media: mgb4: Add support for V4L2_CAP_TIMEPERFRAME

Recent mgb4 firmwares have support for setting a variable framerate
independent of the signal framerate. Add/fix (the mgb4 driver already did
promote V4L2_CAP_TIMEPERFRAME, but it didn't work) support for
V4L2_CAP_TIMEPERFRAME to the driver to enable this feature.

Additionally add support for the DV timings API (VIDIOC_G_DV_TIMINGS,
VIDIOC_ENUM_DV_TIMINGS, ...) for the outputs that was missing. The timings
info is required/used for implementing the V4L2_CAP_TIMEPERFRAME
functionality.

Signed-off-by: Martin Tůma <martin.tuma@digiteqautomotive.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>