sh: clock div6 helper code

This patch adds div6 clock helper code. The div6 clocks
are simply 6-bit divide-by-n modules where n is 1 to 64.

Needed for vclk on sh7722, sh7723, sh7343 and sh7366.
sh7724 needs this even more for vclk, fclka, fclkb,
irdaclk and spuclk.

Signed-off-by: Magnus Damm <damm@igel.co.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>