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

This introduces support for the BGRX8888 input format for the frontend,
with its associated pixel sequence value definition. Other fields are
already configured correctly as they no longer depend on the format's
fourcc directly.

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/20181123092515.2511-22-paul.kocialkowski@bootlin.com