arm: sunxi: Add useful information about sunxi clocks

This patch contains useful bits of information about the sunxi clocks
that may help and/or be interesting for current and future developers.

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