clk: sunxi: fix A20 PLL4 calculation

Allwinner actually reworked the PLL4 on A20; now it's compatible with
the sun4i PLL5/6 design previous to any divisions, as well as to the new
PLL8 in sun7i.

Signed-off-by: Emilio López <emilio@elopez.com.ar>
Signed-off-by: Mike Turquette <mturquette@linaro.org>