media: uapi: add MEDIA_BUS_FMT_BGR666_1X18

Add the BGR666 format MEDIA_BUS_FMT_BGR666_1X18 supported by the
RaspberryPi.

Signed-off-by: Joerg Quinten <aBUGSworstnightmare@gmail.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Link: https://lore.kernel.org/r/20221013-rpi-dpi-improvements-v3-2-eb76e26a772d@cerno.tech


Signed-off-by: Maxime Ripard <maxime@cerno.tech>