sh: shared div4 clock code

Add shared code for 4-bit divisor clocks.

Processor specific code can use SH_CLK_DIV4()
to initialize div4 clocks, and then use
sh_clk_div4_register() for registration.

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