media: uapi: Add MEDIA_BUS_FMT_RGB888_3X8 media bus format

This patch adds MEDIA_BUS_FMT_RGB888_3X8, used for the GiantPlus
GPM940B0 24-bit TFT panel, where the RGB components are transferred
sequentially on a 8-bit bus.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Acked-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190605222247.25657-2-paul@crapouillou.net