arm: dts: mt2701: Use real clock for UARTs

We used to use a fixed rate clock for the UARTs. Now that we have clock
support we can associate the correct clocks to the UARTs and drop the
26MHz fixed rate UART clock.

Signed-off-by: Erin Lo <erin.lo@mediatek.com>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>