dt-bindings: display/panel: Add the Focaltech gpt3

Add bindings for the Forcaltech gpt3, which is a 640x480 3.0" 4:3
IPS LCD Panel found in the YLM/Anbernic RG300X handheld.
Signed-off-by: NChristophe Branchereau <cbranchereau@gmail.com>
Reviewed-by: NKrzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20221219195233.375637-3-cbranchereau@gmail.com

Documentation/devicetree/bindings/display/panel/focaltech,gpt3.yaml

+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/display/panel/focaltech,gpt3.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Focaltech GPT3 3.0" (640x480 pixels) IPS LCD panel
+
+maintainers:
+  - Christophe Branchereau <cbranchereau@gmail.com>
+
+allOf:
+  - $ref: panel-common.yaml#
+  - $ref: /schemas/spi/spi-peripheral-props.yaml#
+
+properties:
+  compatible:
+    const: focaltech,gpt3
+
+  reg:
+    maxItems: 1
+
+required:
+  - compatible
+  - reg
+  - power-supply
+  - reset-gpios
+
+unevaluatedProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/gpio/gpio.h>
+
+    spi {
+        #address-cells = <1>;
+        #size-cells = <0>;
+
+        panel@0 {
+            compatible = "focaltech,gpt3";
+            reg = <0>;
+
+            spi-max-frequency = <3125000>;
+
+            reset-gpios = <&gpe 2 GPIO_ACTIVE_LOW>;
+
+            backlight = <&backlight>;
+            power-supply = <&vcc>;
+
+            port {
+                panel_input: endpoint {
+                    remote-endpoint = <&panel_output>;
+                };
+            };
+        };
+    };