drivers: meson: vdec: add VP9 support to GXM

VP9 support for GXM appears to have been missed from the original
codec submission [0] but it works well, so let's add support.

[0] https://github.com/torvalds/linux/commit/00c43088aa680989407b6afbda295f67b3f123f1



Signed-off-by: Christian Hewitt <christianshewitt@gmail.com>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>