media: mtk-vcodec: venc: specify bitrate range per-chip

Different chips have different supported bitrate ranges. Move the min
and max supported bitrates to the platform data.

Signed-off-by: Alexandre Courbot <acourbot@chromium.org>
Acked-by: Tiffany Lin <tiffany.lin@mediatek.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>