clk: sunxi: add PLL5 and PLL6 support

This commit implements PLL5 and PLL6 support on the sunxi clock driver.
These PLLs use a similar factor clock, but differ on their outputs.

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