media: imx: imx7_mipi_csis: Update MAINTAINERS

Given my recent contributions to the imx7-mipi-csis driver, I can as
well be listed as a maintainer.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Rui Miguel Silva <rmfrfs@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>