drivers: media: pisp_be: Add back V4L2_PIX_FMT_RPI_BE format

Add the opaque V4L2_PIX_FMT_RPI_BE format back to the format list as it
is needed for the verification test suite. Also set the default format
to YUV420 non-multiplanar.

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