media: Documentation: dt-bindings: media: add AST2600 Video Engine support

The AST2600 has Video Engine so add the compatible string into the
document.

Signed-off-by: Jae Hyun Yoo <jae.hyun.yoo@linux.intel.com>
Acked-by: Joel Stanley <joel@jms.id.au>
Acked-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>