ARM: sun8i: a23/a33: Enable the real LOSC and use it

So far, the LOSC was generated through the RTC internal oscillator, which
was a pretty poor and inaccurate choice.

Now that the RTC properly exposes its internal mux between its oscillator
and the external oscillator, we can use it were relevant.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>