ARM: sunxi: dt: Use clocks property instead of clock-frequency for the UARTs

It will be especially useful when we will have the clock definitions in
the device tree.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Emilio López <emilio@elopez.com.ar>
Tested-by: Emilio López <emilio@elopez.com.ar>