drm/vc4: Add support for the VEC (Video Encoder) IP

The VEC IP is a TV DAC, providing support for PAL and NTSC standards.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
(cherry picked from commit e4b81f8c)