sh: shared mstp32 clock code

Add shared 32-bit module stop bit clock support.

Processor specific code can use SH_CLK_MSTP32()
to initialize module stop bit clocks, and then
use sh_clk_mstp32() for registration.

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