dt-bindings: mtk-uart: add mt6765 uart bindings

Add documentation for mt6765 uart dt-bindings

Signed-off-by: Mars Cheng <mars.cheng@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>