clk: sunxi: register factors clocks behind composite

This commit reworks factors clock registration to be done behind a
composite clock. This allows us to additionally add a gate, mux or
divisors, as it will be needed by some future PLLs.

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