clk: sunxi-ng: div: Add support for fixed post-divider

SATA clock on sun4i/sun7i is of type (parent) / M / 6 where
6 is fixed post-divider.

Signed-off-by: Priit Laes <plaes@plaes.org>
Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Chen-Yu Tsai <wens@csie.org>