dt-bindings: display: st7701: Add Anbernic RG28XX panel

The RG28XX panel is a display panel of the Anbernic RG28XX, a handheld
gaming device from Anbernic. It is 2.8 inches in size (diagonally) with
a resolution of 480x640.

This panel is driven by a variant of the ST7701 driver IC internally,
confirmed by dumping and analyzing its BSP initialization sequence
by using a logic analyzer. It is very similar to the existing
densitron,dmt028vghmcmi-1a panel, but differs in some unknown
register values, so add a new entry for the panel to distinguish them.

Additionally, the panel only has an SPI instead of MIPI DSI.
So add and modify for SPI as well.

Although a panel connected via SPI with a D/CX pin theoretically exists,
no such panels have been found for this driver yet. Therefore, disable
the use of the dc-gpios property for all currently known devices.

Signed-off-by: Hironori KIKUCHI <[e-mail]>
Reviewed-by: Conor Dooley <[e-mail]>
Reviewed-by: Neil Armstrong <[e-mail]>
Reviewed-by: Jessica Zhang <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Neil Armstrong <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

Documentation/devicetree/bindings/display/panel/sitronix,st7701.yaml

···
   Densitron DMT028VGHMCMI-1A is 480x640, 2-lane MIPI DSI LCD panel
   which has built-in ST7701 chip.
 
-allOf:
-  - $ref: panel-common.yaml#
-
 properties:
   compatible:
     items:
       - enum:
           - anbernic,rg-arc-panel
+          - anbernic,rg28xx-panel
           - densitron,dmt028vghmcmi-1a
           - elida,kd50t048a
           - techstar,ts8550b
       - const: sitronix,st7701
 
   reg:
-    description: DSI virtual channel used by that screen
+    description: DSI / SPI channel used by that screen
     maxItems: 1
 
   VCC-supply:
···
 
   IOVCC-supply:
     description: I/O system regulator
+
+  dc-gpios:
+    maxItems: 1
+    description:
+      Controller data/command selection (D/CX) in 4-line SPI mode.
+      If not set, the controller is in 3-line SPI mode.
+      Disallowed for DSI.
 
   port: true
   reset-gpios: true
···
   - port
   - reset-gpios
 
-additionalProperties: false
+allOf:
+  - $ref: panel-common.yaml#
+  - if:
+      properties:
+        compatible:
+          contains:
+            # SPI connected panels
+            enum:
+              - anbernic,rg28xx-panel
+    then:
+      $ref: /schemas/spi/spi-peripheral-props.yaml#
+
+  - if:
+      properties:
+        compatible:
+          not:
+            contains:
+              # DSI or SPI without D/CX pin
+              enum:
+                - anbernic,rg-arc-panel
+                - anbernic,rg28xx-panel
+                - densitron,dmt028vghmcmi-1a
+                - elida,kd50t048a
+                - techstar,ts8550b
+    then:
+      required:
+        - dc-gpios
+    else:
+      properties:
+        dc-gpios: false
+
+unevaluatedProperties: false
 
 examples:
   - |
···
             port {
                 mipi_in_panel: endpoint {
                     remote-endpoint = <&mipi_out_panel>;
+                };
+            };
+        };
+    };
+  - |
+    #include <dt-bindings/gpio/gpio.h>
+
+    spi {
+        #address-cells = <1>;
+        #size-cells = <0>;
+
+        panel@0 {
+            compatible = "anbernic,rg28xx-panel", "sitronix,st7701";
+            reg = <0>;
+            spi-max-frequency = <3125000>;
+            VCC-supply = <&reg_lcd>;
+            IOVCC-supply = <&reg_lcd>;
+            reset-gpios = <&pio 8 14 GPIO_ACTIVE_HIGH>; /* LCD-RST: PI14 */
+            backlight = <&backlight>;
+
+            port {
+                panel_in_rgb: endpoint {
+                    remote-endpoint = <&tcon_lcd0_out_lcd>;
                 };
             };
         };