clk: sunxi: get divs parent clock name from parent factor clock

Divs clocks consist of a parent factor clock with multiple outputs,
and seperate clocks for each output. Get the name of the parent
clock from the parent factor clock, instead of the DT node name.

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