media: dt-bindings: media: Add i.MX Pixel Pipeline binding

Add DT binding documentation for the Pixel Pipeline (PXP) found on
various NXP i.MX SoCs.

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