clk: sunxi: protect core clocks from accidental shutdown

Some important clocks may get disabled as a side effect of another clock
being disabled, because they have no consumers. This patch implements a
mechanism so those clocks can be claimed by the driver and therefore
remain enabled at all times.

Signed-off-by: Emilio López <emilio@elopez.com.ar>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>