media: uapi: Add MEDIA_BUS_FMT_BGR888_3X8 media bus format

This patch adds MEDIA_BUS_FMT_BGR888_3X8 used by STM MIPID02 CSI-2 to
PARALLEL bridge driver when input format is MEDIA_BUS_FMT_BGR888_1X24.

Signed-off-by: Mickael Guene <mickael.guene@st.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>