dt-bindings: arm: Document compatibles for Ux500 boards

The device-specific compatible values used by the Ux500 boards
were not documented so far. Add a new simple schema to document them.

Signed-off-by: Stephan Gerhold <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Linus Walleij <[e-mail]>

Documentation/devicetree/bindings/arm/ux500.yaml

···
+# SPDX-License-Identifier: GPL-2.0-only
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/arm/ux500.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Ux500 platforms device tree bindings
+
+maintainers:
+  - Linus Walleij <[e-mail]>
+
+properties:
+  $nodename:
+    const: '/'
+  compatible:
+    oneOf:
+
+      - description: ST-Ericsson HREF (pre-v60)
+        items:
+          - const: st-ericsson,mop500
+          - const: st-ericsson,u8500
+
+      - description: ST-Ericsson HREF (v60+)
+        items:
+          - const: st-ericsson,hrefv60+
+          - const: st-ericsson,u8500
+
+      - description: Calao Systems Snowball
+        items:
+          - const: calaosystems,snowball-a9500
+          - const: st-ericsson,u9500

MAINTAINERS

···
 F:	drivers/watchdog/coh901327_wdt.c
 F:	Documentation/devicetree/bindings/arm/ste-*
 F:	Documentation/devicetree/bindings/arm/ux500/
+F:	Documentation/devicetree/bindings/arm/ux500.yaml
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/linusw/linux-nomadik.git
 
 ARM/NUVOTON NPCM ARCHITECTURE