media: v4l2-common: Add support for fractional bpp

Fraction bytes-per-pixel exist for some packed format. You will find
notably on Rockhip platform that 10bit data is stored fully packed,
meaning that there is 1.25 pixels per bytes. This can be represented
with the fraction 5/4 and can be used to scale the width into a
bytesperline.

Signed-off-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>