dt-bindings: phy: Add STM32MP25 COMBOPHY bindings

Document the bindings for STM32 COMBOPHY interface, used to support
the PCIe and USB3 stm32mp25 drivers.
Following entries can be used to tune caracterisation parameters
- st,output-micro-ohms and st,output-vswing-microvolt bindings entries
to tune the impedance and voltage swing using discrete simulation results
- st,rx-equalizer register to set the internal rx equalizer filter value.

Reviewed-by: Rob Herring (Arm) <[e-mail]>
Signed-off-by: Christian Bruel <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Vinod Koul <[e-mail]>

Documentation/devicetree/bindings/phy/st,stm32mp25-combophy.yaml

···
+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/phy/st,stm32mp25-combophy.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: STMicroelectronics STM32MP25 USB3/PCIe COMBOPHY
+
+maintainers:
+  - Christian Bruel <[e-mail]>
+
+description:
+  Single lane PHY shared (exclusive) between the USB3 and PCIe controllers.
+  Supports 5Gbit/s for USB3 and PCIe gen2 or 2.5Gbit/s for PCIe gen1.
+
+properties:
+  compatible:
+    const: st,stm32mp25-combophy
+
+  reg:
+    maxItems: 1
+
+  "#phy-cells":
+    const: 1
+
+  clocks:
+    minItems: 2
+    items:
+      - description: apb Bus clock mandatory to access registers.
+      - description: ker Internal RCC reference clock for USB3 or PCIe
+      - description: pad Optional on board clock input for PCIe only. Typically an
+                     external 100Mhz oscillator wired on dedicated CLKIN pad. Used as reference
+                     clock input instead of the ker
+
+  clock-names:
+    minItems: 2
+    items:
+      - const: apb
+      - const: ker
+      - const: pad
+
+  resets:
+    maxItems: 1
+
+  reset-names:
+    const: phy
+
+  power-domains:
+    maxItems: 1
+
+  wakeup-source: true
+
+  interrupts:
+    maxItems: 1
+    description: interrupt used for wakeup
+
+  access-controllers:
+    maxItems: 1
+    description: Phandle to the rifsc device to check access right.
+
+  st,ssc-on:
+    $ref: /schemas/types.yaml#/definitions/flag
+    description:
+      A property whose presence indicates that the Spread Spectrum Clocking is active.
+
+  st,rx-equalizer:
+    $ref: /schemas/types.yaml#/definitions/uint32
+    minimum: 0
+    maximum: 7
+    default: 2
+    description:
+      A 3 bit value to tune the RX fixed equalizer setting for optimal eye compliance
+
+  st,output-micro-ohms:
+    minimum: 3999000
+    maximum: 6090000
+    default: 4968000
+    description:
+      A value property to tune the Single Ended Output Impedance, simulations results
+      at 25C for a VDDP=0.8V. The hardware accepts discrete values in this range.
+
+  st,output-vswing-microvolt:
+    minimum: 442000
+    maximum: 803000
+    default: 803000
+    description:
+      A value property in microvolt to tune the Single Ended Output Voltage Swing to change the
+      Vlo, Vhi for a VDDP = 0.8V. The hardware accepts discrete values in this range.
+
+required:
+  - compatible
+  - reg
+  - "#phy-cells"
+  - clocks
+  - clock-names
+  - resets
+  - reset-names
+
+additionalProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/clock/st,stm32mp25-rcc.h>
+    #include <dt-bindings/interrupt-controller/arm-gic.h>
+    #include <dt-bindings/reset/st,stm32mp25-rcc.h>
+
+    phy@480c0000 {
+        compatible = "st,stm32mp25-combophy";
+        reg = <0x480c0000 0x1000>;
+        #phy-cells = <1>;
+        clocks = <&rcc CK_BUS_USB3PCIEPHY>, <&rcc CK_KER_USB3PCIEPHY>;
+        clock-names = "apb", "ker";
+        resets = <&rcc USB3PCIEPHY_R>;
+        reset-names = "phy";
+        access-controllers = <&rifsc 67>;
+        power-domains = <&CLUSTER_PD>;
+        wakeup-source;
+        interrupts-extended = <&exti1 45 IRQ_TYPE_EDGE_FALLING>;
+    };