ARM: dts: mediatek mt8135: Use real UART clocks

We used to provide dummy clocks for the UART. Now that we have
common clock support we can provide the real clocks to the UART.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>