sh: Wire up GENERIC_CMOS_UPDATE for the platforms that need it.

Now that everything has converted over to generic timekeeping, we need an
alternate method for keeping the RTC updated for those platforms that are
still using the rtc_sh_get/set_time pairs, presently limited to SH-03 and
the Dreamcast. This wires up the GENERIC_CMOS_UPDATE hooks for those to
maintain the same behaviour.

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