IB/pma: Add include file for IBA performance counters definitions

Move the various definitions and mad structures needed for software
implementation of IBA PM agent from the ipath and qib drivers into a
single include file, which in turn could be used by more consumers.

Signed-off-by: Or Gerlitz <ogerlitz@mellanox.co.il>
Signed-off-by: Roland Dreier <roland@purestorage.com>