Among the many entries in the TSB12LV22 errata list (TI literature
number SLLS312) is the following:

  PCI Slave reads of the Cycle Timer register may occasionally get an
  incorrect value.
  Software may be able to validate value by reading the register
  multiple times rapidly and evaluating for a reasonable difference.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de> (untested)
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de> (added #define)