alpha: Reorganize rtc handling

Discontinue use of GENERIC_CMOS_UPDATE; rely on the RTC subsystem.

The marvel platform requires that the rtc only be touched from the
boot cpu.  This had been partially implemented with hooks for
get/set_rtc_time, but read/update_persistent_clock were not handled.
Move the hooks from the machine_vec to a special rtc_class_ops struct.

We had read_persistent_clock managing the epoch against which the
rtc hw is based, but this didn't apply to get_rtc_time or set_rtc_time.
This resulted in incorrect values when hwclock(8) gets involved.

Allow the epoch to be set from the kernel command-line, overriding
the autodetection, which is doomed to fail in 2020.  Further, by
implementing the rtc ioctl function, we can expose this epoch to
userland.

Elide the alarm functions that RTC_DRV_CMOS implements.  This was
highly questionable on Alpha, since the interrupt is used by the
system timer.

Signed-off-by: Richard Henderson <rth@twiddle.net>

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