[media] coda: move H.264 helper function into separate file

Currently there is only the coda_h264_padding function, but
we will have to add more H.264 specific helpers later.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>