dt-bindings: arm: arm,juno-fpga-apb-regs: document FPGA syscon

Add dedicated bindings for the FPGA syscon registers on ARM Juno board,
to fully document the block and also fix dtbs_check warning:

juno.dtb: apbregs@10000: compatible: ['syscon', 'simple-mfd'] is too short

Signed-off-by: Krzysztof Kozlowski <[e-mail]>
Reviewed-by: Conor Dooley <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Sudeep Holla <[e-mail]>

Documentation/devicetree/bindings/arm/arm,juno-fpga-apb-regs.yaml

···
+# SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/arm/arm,juno-fpga-apb-regs.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: ARM Juno FPGA APB Registers
+
+maintainers:
+  - Sudeep Holla <[e-mail]>
+
+properties:
+  compatible:
+    items:
+      - const: arm,juno-fpga-apb-regs
+      - const: syscon
+      - const: simple-mfd
+
+  reg:
+    maxItems: 1
+
+  ranges: true
+
+  "#address-cells":
+    const: 1
+
+  "#size-cells":
+    const: 1
+
+patternProperties:
+  "^led@[0-9a-f]+,[0-9a-f]$":
+    $ref: /schemas/leds/register-bit-led.yaml#
+
+required:
+  - compatible
+  - reg
+  - ranges
+  - "#address-cells"
+  - "#size-cells"
+
+additionalProperties: false
+
+examples:
+  - |
+    syscon@10000 {
+        compatible = "arm,juno-fpga-apb-regs", "syscon", "simple-mfd";
+        reg = <0x010000 0x1000>;
+        ranges = <0x0 0x10000 0x1000>;
+        #address-cells = <1>;
+        #size-cells = <1>;
+
+        led@8,0 {
+            compatible = "register-bit-led";
+            reg = <0x08 0x04>;
+            offset = <0x08>;
+            mask = <0x01>;
+            label = "vexpress:0";
+            linux,default-trigger = "heartbeat";
+            default-state = "on";
+        };
+    };