o Don't announce that the timer is good when in fact it isn't timing

o  Don't announce that the timer is good when in fact it isn't timing
   at all (ie reads yield constant values). Display the width as the
   difference between max and min so that constant timers have width
   zero.
o  Get the address of the timer from the XPmTmrBlk field instead of
   the V1_PmTmrBlk field. The former is a generic address and can
   specify a memory mapped I/O address. Remove <machine/bus_pio.h>
   to account for this. The timer is now properly configured on
   machines with ACPI v2 tables, whether PIO or MEMIO. Note that
   the acpica code converts v1 tables into v2 tables so the address
   is always present in XPmTmrBlk.
o  Replace the TIMER_READ macro with a call to the read_counter()
   function and add a barrier to make sure that we observe proper
   ordering of the reads.