sh: clkfwk: Support variable size accesses for div4/div6 clocks.

This follows the MSTP clock change and implements variable access size
support for the rest of the CPG clocks, too. Upcoming SH-2A support has
need of this for 16-bit div4 clocks, while others will follow.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>