media: imx-pxp: add i.MX Pixel Pipeline driver

Add a V4L2 mem-to-mem scaler/CSC driver for the Pixel Pipeline (PXP)
version found on i.MX6ULL SoCs. A similar variant is used on i.MX7D.

Since this driver only uses the legacy pipeline, it should be reasonably
easy to extend it to work with the older PXP versions found on i.MX6UL,
i.MX6SX, i.MX6SL, i.MX28, and i.MX23.

The driver supports scaling and colorspace conversion. There is
currently no support for rotation, alpha-blending, and the LUTs.

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