media: imx-pxp: Introduce pxp_read() and pxp_write() wrappers

Add pxp_read() and pxp_write() functions to wrap readl() and writel()
respectively. This can be useful for debugging register accesses.

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