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

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 <kikuchan98@gmail.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Reviewed-by: Jessica Zhang <quic_jesszhan@quicinc.com>
Link: https://lore.kernel.org/r/20240804061503.881283-4-kikuchan98@gmail.com
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240804061503.881283-4-kikuchan98@gmail.com