[media] mx2_camera: Convert it to platform driver

Converting it to platform code can make the code smaller.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>