[media] v4l: videobuf2: add DMA scatter/gather allocator

Add an implementation of DMA scatter/gather allocator and handling
routines for videobuf2.

For mmap operation mode it is implemented on top of
alloc_page + sg_set_page/_free_page.

For userptr operation mode it is implemented on top of
get_user_pages + sg_set_page/put_page.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
CC: Pawel Osciak <pawel@osciak.com>
Reviewed-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>