Make the ARM MPCore Timer driver work with published standard FDT bindings.

We've always considered the mpcore timers to be a single monolithic device
and we defined our own fdt binding for it with our own compat string.  The
published bindings treat the timers as two separate devices, a global
timer and a "timer-watchdog" device for the per-cpu private timers.  Thus
our binding has two tuples in the regs property, one set of registers for
the global timer and one for the private timers.  The published bindings
have two separate devices, each with a single set of registers.  (Note that
we don't use the optional watchdog feature of the hardware.)

These changes add the compat strings for the published bindings.  If our
own compat string appears, we expect to get two sets of memory resources.
For the published bindings, there's only one set of memory resources, and
only the private timers have an associated interrupt.

The other major change is that there can no longer be a single global var
for the softc pointer because now there may be multiple devices at
runtime.  Since the global timer is used only as a timecounter and the
private timers only as eventtimers, and there will only be one of each,
those are now the pointers which are global, and the priv fields of those
structures backlink to the device softc.