media: hantro: Add core bits to support H264 decoding

Add helpers and patch hantro_{drv,v4l2}.c to prepare addition of H264
decoding support.

Signed-off-by: Hertz Wong <hertz.wong@rock-chips.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Tested-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>