dt-bindings: display: Add Arm virtual platforms display

'arm,rtsm-display' is a panel for Arm, Ltd. virtual platforms (e.g. FVP).
The binding has been in use for a long time, but was never documented.

Some users and an example have a 'panel-dpi' compatible, but that's not
needed without a 'panel-timing' node which none of the users have since
commit 928faf5e ("arm64: dts: fvp: Remove panel timings"). The
example does have a 'panel-timing' node, but it should not for the
same reasons the node was removed in the dts files. So update the
example in arm,pl11x.yaml to match the schema.

Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Robin Murphy <robin.murphy@arm.com>
Cc: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20220613145709.3729053-1-robh@kernel.org

Documentation/devicetree/bindings/display/arm,pl11x.yaml

@@ -159,25 +159,12 @@ examples:
     };
 
     panel {
-        compatible = "arm,rtsm-display", "panel-dpi";
-        power-supply = <&vcc_supply>;
+        compatible = "arm,rtsm-display";
 
         port {
             clcd_panel: endpoint {
                 remote-endpoint = <&clcd_pads>;
             };
         };
-
-        panel-timing {
-            clock-frequency = <25175000>;
-            hactive = <640>;
-            hback-porch = <40>;
-            hfront-porch = <24>;
-            hsync-len = <96>;
-            vactive = <480>;
-            vback-porch = <32>;
-            vfront-porch = <11>;
-            vsync-len = <2>;
-        };
     };
 ...

Documentation/devicetree/bindings/display/panel/arm,rtsm-display.yaml

+# SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/display/panel/arm,rtsm-display.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Arm RTSM Virtual Platforms Display
+
+maintainers:
+  - Linus Walleij <linus.walleij@linaro.org>
+
+allOf:
+  - $ref: panel-common.yaml#
+
+properties:
+  compatible:
+    const: arm,rtsm-display
+
+  port: true
+
+required:
+  - compatible
+  - port
+
+additionalProperties: false
+
+...