clk: sunxi: Make divs clocks specify which output is the base factor clock

The current sunxi clock driver has the base factor clock of divs clocks
as the last clock output of the clock node. This makes it rather difficult
to add new outputs, such as fixed dividers, which were previously unknown.

This patch makes the divs clocks data structure specify which output is
the factor clock, and updates all current divs clocks accordingly.

We can then add new outputs after the factor clocks, at least not breaking
backward compatibility with regards to the devicetree bindings.

Also replace kzalloc with kcalloc in sunxi_divs_clk_setup().

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>