dt-bindings: serio: add Arm PL050 DT schema

The Arm PL050 "Keyboard/Mouse Interface" is an Arm system IP providing a
PS/2 compatible serial interface.

Add a simple DT schema binding, based on the TRM[1], the existing DTs and
the Linux driver.

[1] https://developer.arm.com/documentation/ddi0143/latestSigned-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220506140533.3566431-6-andre.przywara@arm.com

Documentation/devicetree/bindings/serio/arm,pl050.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/serio/arm,pl050.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Arm Ltd. PrimeCell PL050 PS/2 Keyboard/Mouse Interface
+
+maintainers:
+  - Andre Przywara <andre.przywara@arm.com>
+
+description:
+  The Arm PrimeCell PS2 Keyboard/Mouse Interface (KMI) is an AMBA compliant
+  peripheral that can be used to implement a keyboard or mouse interface that
+  is IBM PS2 or AT compatible.
+
+# We need a select here so we don't match all nodes with 'arm,primecell'
+select:
+  properties:
+    compatible:
+      contains:
+        const: arm,pl050
+  required:
+    - compatible
+
+properties:
+  compatible:
+    items:
+      - const: arm,pl050
+      - const: arm,primecell
+
+  reg:
+    maxItems: 1
+
+  interrupts:
+    maxItems: 1
+
+  clocks:
+    items:
+      - description: KMI reference clock, used to generate the bus timing
+      - description: APB register access clock
+
+  clock-names:
+    items:
+      - const: KMIREFCLK
+      - const: apb_pclk
+
+required:
+  - compatible
+  - reg
+  - interrupts
+  - clocks
+  - clock-names
+
+additionalProperties: false
+
+examples:
+  - |
+    serio@70000 {
+        compatible = "arm,pl050", "arm,primecell";
+        reg = <0x070000 0x1000>;
+        interrupts = <8>;
+        clocks = <&mb_clk24mhz>, <&soc_smc50mhz>;
+        clock-names = "KMIREFCLK", "apb_pclk";
+    };
+
+...