media: dt-bindings: media: amlogic,gx-vdec: add bindings for G12A family

Add bindings to support the Amlogic Video Decoder on the Amlogic G12A
family.

For the G12A family, a supplementary clock is needed to operate the
HEVC/VP9 decoder.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>