media: bcm2835-unicam: Add support for luma/greyscale formats

Add support for V4L2_PIX_FMT_GREY, V4L2_PIX_FMT_Y10P,
V4L2_PIX_FMT_Y10, and V4L2_PIX_FMT_Y12 image formats from the
appropriate mediabus formats advertised by sensors.

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