media: imx-pxp: explicitly disable unused blocks

Various multiplexers in the pipeline are not used with the currently
configured data path. Disable all unused multiplexers by selecting the
"no output" (3) option.

The datasheet doesn't explicitly require this, but the PXP has been seen
to hang after processing a few hundreds of frames otherwise.

As at it, add documentation for the multiplexers that are actually
relevant for the data path.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>