sh: dreamcast: rtc: push down rtc class ops into driver

The SH RTC support has an extra level of indirection to provide
either the old read_persistent_clock/update_persistent_clock
interface or the rtc-generic device for hctosys/systohc.

Both do the same thing for dreamcast, so we can do away with the
abstraction and simply let the RTC core code to take care of it.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>