sh: clkfwk: Use shared sh_clk_div_enable/disable().

This introduces a new flag for clocks that need to have their divisor
ratio set back to their initial mask at disable time to prevent
interactivity problems with the clock stop bit (presently div6 only).
With this in place it's possible to handle the corner case on top of the
div4 op without any particular need for leaving things split out.

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