clk: sunxi-ng: nkm: add support for fixed post-divider

SATA PLL on Allwinner R40 is of type (parent) * N * K / M / 6 where 6 is
the fixed post-divider.

Add post-divider support for NKM type clock.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
[wens@csie.org: Fixed application of post-divider in set_rate callback]
Signed-off-by: Chen-Yu Tsai <wens@csie.org>