[media] media: venus: enable building of Venus video driver

This adds Venus driver Makefile and changes v4l2 platform
Makefile/Kconfig in order to enable building of the driver.

Note that in this initial version the COMPILE_TEST-ing is not
supported because the drivers specific to ARM builds are still
in process of enabling the aforementioned compile testing.
Once that disadvantage is fixed the Venus driver compile testing
will be possible with follow-up changes.

Signed-off-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>