media: mediatek: vcodec: Read HW active status from syscon

Remove the requirement of a VDEC_SYS reg iospace for both MT8173 and
MT8183. To achieve that, rely on a vdecsys syscon to be passed through
the DT, and use it to directly read the VDEC_HW_ACTIVE bit during IRQ
handling to check whether the HW is active. Also update the VP8 stateful
decoder to use the syscon, if present, for writes to VDEC_SYS.

The old behavior is still present when reg-names aren't supplied, as
to keep backward compatibility.

Signed-off-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
[hverkuil: added vdecsys_regmap kerneldoc line provided by Nicolas]