dt-bindings: pwrap: mediatek: add pwrap support for MT6779

Add binding document of pwrap for MT6779 SoCs.

Signed-off-by: Argus Lin <argus.lin@mediatek.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>