[media] saa7134: add saa7134_userptr module option to enable USERPTR

If the saa7134 module is loaded with the saa7134_userptr set to 1,
then USERPTR support is enabled. A check in buffer_prepare
verifies that the pointer is page-aligned.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>