dt-bindings: arm: mediatek: add MT7622 string to the PMIC wrapper doc

Signed-off-by: Chenglin Xu <chenglin.xu@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>