staging: bcm2835-codec: Add support for V4L2_CID_MPEG_VIDEO_GOP_SIZE

For H264, V4L2_CID_MPEG_VIDEO_H264_I_PERIOD is meant to be the intra
I-frame period, whilst V4L2_CID_MPEG_VIDEO_GOP_SIZE is the intra IDR
frame period.
The firmware encoder doesn't produce I-frames that aren't IDR as well,
therefore V4L2_CID_MPEG_VIDEO_GOP_SIZE is technically the correct
control, however users may have adopted V4L2_CID_MPEG_VIDEO_H264_I_PERIOD.

Add support for V4L2_CID_MPEG_VIDEO_GOP_SIZE controlling the encoder,
and have VIDIOC_S_CTRL for V4L2_CID_MPEG_VIDEO_H264_I_PERIOD update
the value for V4L2_CID_MPEG_VIDEO_GOP_SIZE (the reverse is not
implemented).

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>