media: dt-bindings: Document SAMA5D4 VDEC bindings

Add devicetree binding documentation for the Hantro G1/G2 VDEC on
the Microchip SAMAS5D4 SoC.

Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.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>