ARM: dts: Add clocks to the Gemini SoC

We have a clock controller for the Gemini SoC, so make use of the
driver and add clocks to the peripherals. Remove the hard-coded
frequency from the UART and add switch the timer compatible to the
generic that uses the clock framework for clock speed look-up.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>