media: rkisp1: Enable compilation on ARCH_MXC

The ISP used by the Rockchip RK3399 is also found in the NXP i.MX8MP.
Enable compilation of the driver for the MXC architecture in addition to
ARCH_ROCKCHIP.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Dafna Hirschfeld <dafna@fastmail.com>
Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>