sh: add enable()/disable()/set_rate() to div6 code

This patch updates the div6 clock helper code to add support
for enable(), disable() and set_rate() callbacks.

Needed by the camera clock enabling board code on Migo-R.

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