dt-bindings: clocks: add binding for gated-fixed-clocks

In contrast to fixed clocks that are described as ungateable, boards
sometimes use additional oscillators for things like PCIe reference
clocks, that need actual supplies to get enabled and enable-gpios to be
toggled for them to work.

This adds a binding for such oscillators that are not configurable
themself, but need to handle supplies for them to work.

In schematics they often can be seen as

----------------
Enable - | 100MHz,3.3V, | - VDD
| 3225 |
GND - | | - OUT
----------------

or similar. The enable pin might be separate but can also just be tied
to the vdd supply, hence it is optional in the binding.

Signed-off-by: Heiko Stuebner <[e-mail]>
Reviewed-by: Rob Herring (Arm) <[e-mail]>
Reviewed-by: Conor Dooley <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Stephen Boyd <[e-mail]>

Documentation/devicetree/bindings/clock/gated-fixed-clock.yaml

···
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/clock/gated-fixed-clock.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Gated Fixed clock
+
+maintainers:
+  - Heiko Stuebner <[e-mail]>
+
+properties:
+  compatible:
+    const: gated-fixed-clock
+
+  "#clock-cells":
+    const: 0
+
+  clock-frequency: true
+
+  clock-output-names:
+    maxItems: 1
+
+  enable-gpios:
+    description:
+      Contains a single GPIO specifier for the GPIO that enables and disables
+      the oscillator.
+    maxItems: 1
+
+  vdd-supply:
+    description: handle of the regulator that provides the supply voltage
+
+required:
+  - compatible
+  - "#clock-cells"
+  - clock-frequency
+  - vdd-supply
+
+additionalProperties: false
+
+examples:
+  - |
+    clock-1000000000 {
+      compatible = "gated-fixed-clock";
+      #clock-cells = <0>;
+      clock-frequency = <1000000000>;
+      vdd-supply = <&reg_vdd>;
+    };
+...