media: imx-pxp: convert to regmap

Replace the readl and writel with regmap to ease debugging the registers
from userspace.

Suggested-by: Alexander Stein <alexander.stein@ew.tq-group.com>
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>