dt-bindings: panel: Add Powkiddy RGB30 panel compatible

The Powkiddy RGB30 panel is a 4 inch 720x720 MIPI-DSI LCD panel. It
appears to be based on the ST7703 LCD controller (this is assumed from
the init sequence similarity between this and other displays). Powkiddy
would not share the part number or name for the display from the bill
of materials and there were no obvious external markings, so name the
panel for the device (Powkiddy RGB30).

Signed-off-by: Chris Morgan <[e-mail]>
Acked-by: Krzysztof Kozlowski <[e-mail]>
Signed-off-by: Heiko Stuebner <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

Documentation/devicetree/bindings/display/panel/rocktech,jh057n00900.yaml

···
     enum:
       # Anberic RG353V-V2 5.0" 640x480 TFT LCD panel
       - anbernic,rg353v-panel-v2
+      # Powkiddy RGB30 3.0" 720x720 TFT LCD panel
+      - powkiddy,rgb30-panel
       # Rocktech JH057N00900 5.5" 720x1440 TFT LCD panel
       - rocktech,jh057n00900
       # Xingbangda XBD599 5.99" 720x1440 TFT LCD panel