V4L/DVB (11242): allow v4l2 drivers to provide a get_unmapped_area handler

Shared memory mappings on nommu machines require a get_unmapped_area
file operation that suggests an address for the mapping. This patch
adds a way for v4l2 drivers to provide this callback.

Signed-off-by: Daniel Glöckner <dg@emlix.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>