drm/sun4i: frontend: Add support for the BGRX8888 output format

This introduces support for the BGRX8888 output format for the frontend,
with its associated output format value definition.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181123092515.2511-23-paul.kocialkowski@bootlin.com