media: uapi: v4l2-core: Add ISP statistics output V4L2 fourcc type

Add V4L2_META_FMT_BCM2835_ISP_STATS V4L2 format type.

This new format will be used by the BCM2835 ISP device to return
out ISP statistics for 3A.

Signed-off-by: Naushir Patuck <naush@raspberrypi.com>