Add RGB666_1X24_CPADHI media bus format

Commit 9e74d292 ("staging: imx-drm: add LVDS666 support for parallel
display") describes a 24-bit bus format where three 6-bit components each
take the lower part of 8 bits with the two high bits zero padded. Add a
component-wise padded media bus format RGB666_1X24_CPADHI to support this
connection.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Tested-by: Emil Renner Berthing <kernel@esmil.dk>