media: imx: use well defined 32-bit RGB pixel format

The documentation in Documentation/media/uapi/v4l/pixfmt-packed-rgb.rst
tells us that the V4L2_PIX_FMT_RGB32 format is deprecated and must not
be used by new drivers. Replace it with V4L2_PIX_FMT_XRGB32.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Steve Longerbeam <steve_longerbeam@mentor.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>