dt-bindings: display: convert lg,lg4573 to DT Schema

v2:
  - Dropped spi-slave (Maxime)
  - Added unevaluatedProperties (Maxime)
  - Deleted needless compatible from example (Rob)
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Rob Herring <robh@kernel.org>
Cc: Heiko Schocher <hs@denx.de>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200408195109.32692-17-sam@ravnborg.org

Documentation/devicetree/bindings/display/panel/lg,lg4573.txt

-LG LG4573 TFT Liquid Crystal Display with SPI control bus
-
-Required properties:
-  - compatible: "lg,lg4573"
-  - reg: address of the panel on the SPI bus
-
-The panel must obey rules for SPI slave device specified in document [1].
-
-[1]: Documentation/devicetree/bindings/spi/spi-bus.txt
-
-Example:
-
-	lcd_panel: display@0 {
-		#address-cells = <1>;
-		#size-cells = <1>;
-		compatible = "lg,lg4573";
-		spi-max-frequency = <10000000>;
-		reg = <0>;
-	};

Documentation/devicetree/bindings/display/panel/lg,lg4573.yaml

+# SPDX-License-Identifier: GPL-2.0
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/display/panel/lg,lg4573.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: LG LG4573 TFT Liquid Crystal Display with SPI control bus
+
+description: |
+  The panel must obey the rules for a SPI slave device as specified in
+  spi/spi-controller.yaml
+
+maintainers:
+  - Heiko Schocher <hs@denx.de>
+
+allOf:
+  - $ref: panel-common.yaml#
+
+properties:
+  compatible:
+    const: lg,lg4573
+
+  reg: true
+  spi-max-frequency: true
+
+required:
+  - compatible
+  - reg
+
+unevaluatedProperties: false
+
+examples:
+  - |
+    spi {
+        #address-cells = <1>;
+        #size-cells = <0>;
+
+        lcd_panel: display@0 {
+            compatible = "lg,lg4573";
+            spi-max-frequency = <10000000>;
+            reg = <0>;
+        };
+    };
+
+...