[media] i.MX27 camera: add support for YUV420 format

This patch uses channel 2 of the eMMa-PrP to convert
format provided by the sensor to YUV420.

This format is very useful since it is used by the
internal H.264 encoder.
Signed-off-by: Javier Martin <javier.martin@vista-silicon.com>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>