[media] V4L: soc-camera: add a new packing for YUV 4:2:0 type formats

12-bit formats, similar to YUV 4:2:0 occupy 3 bytes for each two pixels
and cannot be described by any of the existing SOC_MBUS_PACKING_* macros.
This patch adds a new one SOC_MBUS_PACKING_1_5X8 to describe such
formats and extends soc_mbus_samples_per_pixel() to support it.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>