dt-bindings: sunxi: Add CCU binding documentation

Introduce a new binding with its documentation for the brand new clock
sub-framework.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>
Link: lkml.kernel.org/r/20160629190535.11855-2-maxime.ripard@free-electrons.com