clk: sunxi: factors: Invert the probing logic

Until now, the factors clock probing was done directly by sunxi_init_clocks,
with the factors registration being called directly with the clocks data passed
as an argument.

This approch has shown its limits when we added more clocks, since we couldn't
really split code with such a logic in smaller files, and led to a huge file
having all the clocks.

Introduce an intermediate probing function, so that factor clocks will be able
to directly be called by CLK_OF_DECLARE, which will in turn ease the split into
several files.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Hans de Goede <hdegoede@redhat.com>