media: mtk-vcodec: Get numbers of register bases from DT

Different platforms may have different numbers of register bases. Gets the
numbers of register bases from dts (sizeof(u32) * 4 bytes for each).

Signed-off-by: Yunfei Dong <yunfei.dong@mediatek.com>
Reviewed-by: Tzung-Bi Shih <tzungbi@google.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>