Allwinner hardware sucks! The ARM generic timer on the A64 has a bug

Allwinner hardware sucks!  The ARM generic timer on the A64 has a bug
where the bottom 9 bits of the counter register can't be trusted if any of
the higher bits are rolling over.  This is an unpublished errata so the
details aren't known.  Adopt the same workaround that Linux has.

This will disable the userland timecounter support on hardware affected
by the hardware.  We will need a similar workaround in libc to restore
that functionality.

tested by semarie@
ok cheloha@