sh: add a reparent function to DIV6 clocks

Add support for reparenting of div6 clocks on SuperH and SH-Mobile SoCs.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Acked-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>