clk: sun6i: Protect CPU clock

Right now, AHB is an indirect child clock of the CPU clock. If that
happens to change, since the CPU clock has no other consumers declared
in Linux, it would be shut down, which is not really a good idea.

Prevent this by forcing it enabled.

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>