powerpc/eeh: Output PHB3 diag-data

The patch adds function ioda_eeh_phb3_phb_diag() to dump PHB3
PHB diag-data. That's called while detecting informative errors
or frozen PE on the specific PHB.

Signed-off-by: Gavin Shan <shangw@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>