clk: sunxi: Unify oscillator clock

This commit uses the new fixed-rate support on the composite clock to
unify osc24M_fixed and osc24M clocks, so it matches the actual hardware.

Signed-off-by: Emilio López <emilio@elopez.com.ar>
Signed-off-by: Mike Turquette <mturquette@linaro.org>
[mturquette@linaro.org: replace clk_register_gatable_osc with a call to
clk_register_composite]