sh: sh03: 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.

By removing the indirection and always using the RTC_CLASS interface,
we can avoid the lossy double conversion between rtc_time and timespec,
so we end up supporting the entire range of 'year' values, and clarifying
the rtc_set_time callback.

I did not change the behavior of sh03_rtc_settimeofday(), which keeps
just updating the seconds/minutes by calling set_rtc_mmss(), this
could be improved if anyone cares. Also, the file should ideally be
moved into drivers/rtc and not use rtc-generic.

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