dt-bindings: Add mtk-timer bindings

Add binding documentation for the General Purpose Timer driver of
the Mediatek SoCs.

Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>
Acked-by: Sören Brinkmann <soren.brinkmann@xilinx.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>