dt-bindings: rtc: mediatek: add bindings for MediaTek SoC based RTC

Add device-tree binding for MediaTek SoC based RTC

Cc: devicetree@vger.kernel.org
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>