clk: sunxi: Move the 24M oscillator to a file of its own

Since we have a folder of our own, we can actually make use of it by
splitting the huge clock file into several sub drivers.

The main oscillator is pretty easy to deal with, since it's pretty much
isolated.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Mike Turquette <mturquette@linaro.org>
Signed-off-by: Emilio López <emilio@elopez.com.ar>