Fix two problems that the time runs late on virt68k.
- The time between the time the alarm occurred and the time read by
TIME_* register in the next interrupt handler was not accumulated.
- With the one-shot timer method, once the host time prolongs, the
guest time will never be able to catch up with the host time again.
New one does:
- The driver maintains its (guest's) time (as sc_alarm_time) and always
set the next alarm sc_interval_ns after the previous alarm.
- gfrtc_set_alarm() takes an absolute time instead of a relative time
as the argument.
PR kern/57980. Confirmed on QEMU.

Fix nanosecond math in gfrtc_gettime().

Add support for the timer portion of the Goldfish RTC device.

gfrtc_settime(): Write the low half of the time valueinto the RTC_TIME_LOW
register, not the RTC_TIME_HIGH register.

Re-factor the Goldfish RTC driver into attach-front-end and generic
back-end; Goldfish virtual devices can be found on virtual platforms
that don't use FDT.

Fix nanosecond math in gfrtc_gettime().

Add support for the timer portion of the Goldfish RTC device.

gfrtc_settime(): Write the low half of the time valueinto the RTC_TIME_LOW
register, not the RTC_TIME_HIGH register.

Re-factor the Goldfish RTC driver into attach-front-end and generic
back-end; Goldfish virtual devices can be found on virtual platforms
that don't use FDT.

Add support for the timer portion of the Goldfish RTC device.

gfrtc_settime(): Write the low half of the time valueinto the RTC_TIME_LOW
register, not the RTC_TIME_HIGH register.

Re-factor the Goldfish RTC driver into attach-front-end and generic
back-end; Goldfish virtual devices can be found on virtual platforms
that don't use FDT.