drm/panel: sitronix-st7789v: add media bus format

Add support for describing the media bus format in the
panel configuration and expose that to userspace. Since
both supported formats (RGB565 and RGB666) are using 6
bits per color also hardcode that information.

Reviewed-by: Michael Riesch <michael.riesch@wolfvision.net>
Signed-off-by: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230714013756.1546769-11-sre@kernel.org