In previous versions of the Arm architecture, the frequency of the
generic timers as reported in CNTFRQ_EL0 could be any IMPDEF value,
and for QEMU we picked 62.5MHz, giving a timer tick period of 16ns.
In Armv8.6, the architecture standardized this frequency to 1GHz.

Because there is no ID register feature field that indicates whether
a CPU is v8.6 or that it ought to have this counter frequency, we
implement this by changing our default CNTFRQ value for all CPUs,
with exceptions for backwards compatibility:

 * CPU types which we already implement will retain the old
   default value. None of these are v8.6 CPUs, so this is
   architecturally OK.
 * CPUs used in versioned machine types with a version of 9.0
   or earlier will retain the old default value.

The upshot is that the only CPU type that changes is 'max'; but any
new type we add in future (whether v8.6 or not) will also get the new
1GHz default.

It remains the case that the machine mo...