arm64: dts: mt8173: add mmsel clocks for 4K support

To support HDMI 4K resolution, mmsys need clcok
mm_sel to be 400MHz.

The board .dts file should override the clock rate
property with the higher VENCPLL frequency the board
supports HDMI 4K resolution.

Signed-off-by: Bibby Hsieh <bibby.hsieh@mediatek.com>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>