sh: clkfwk: Move to common clk_div_table accessors for div4/div6.

This plugs in a generic clk_div_table, based on the div4 version. div6 is
then adopted to use it for encapsulating its div table, which permits us
to start div6/4 unification, as well as preparation for other div types.

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