Use correct offset for the high byte of high memory written to
RTC NVRAM.

Submitted by:	Bela Lubkin   bela dot lubkin at tidalscale dot com
Approved by:	re@ (blanket)
Allow the alarm hours/mins/seconds registers to be read/written,
though without any action. This avoids a hypervisor exit when
o/s's access these regs (Linux).

Reviewed by:	neel
Approved by:	re@ (blanket)
Add simplistic periodic timer support to mevent using kqueue's
timer support. This should be enough for the emulation of
h/w periodic timers (and no more) e.g. some of the 8254's
more esoteric modes that happen to be used by non-FreeBSD o/s's.

Approved by:	re@ (blanket)
Implement support for the interrupt-on-terminal-count and
s/w-strobe timer modes. These are commonly used by non-FreeBSD
o/s's.

Approved by:	re@ (blanket)