powerpc/powernv: Handle compound PE

The patch introduces 3 PHB callbacks: compound PE state retrieval,
force freezing and unfreezing compound PE. The PCI config accessors
and PowerNV EEH backend can use them in subsequent patches.

We don't export the capability of compound PE to EEH core, which
helps avoiding more complexity to EEH core.

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