dt-bindings: mediatek: add compatible for MT6873/8192 pwrap

This adds dt-binding documentation of pwrap for Mediatek MT6873/8192
SoCs Platform.

Signed-off-by: Hsin-Hsiung Wang <hsin-hsiung.wang@mediatek.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1615563286-22126-4-git-send-email-hsin-hsiung.wang@mediatek.com


Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>