rtc: rtc-sh: Split out the CPU defs to asm/cpu/.

With all of the different CPU types this was getting a but unwieldly.
Since sh64 is now integrated, we don't have to worry about multiple
architectures caring about the header definitions.

Split out the defs for each asm/cpu/ to make rtc-sh slightly less
visually offensive.

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