Add BGR888_1X24 and GBR888_1X24 media bus formats

This patch adds two more 24-bit RGB formats. BGR888 is more or less common,
GBR888 is used on the internal connection between the IPU display interface
and the TVE (VGA DAC) on i.MX53 SoCs.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>