powerpc/pseries: Add support for ibm,req#msi-x

Firmware encodes the number of MSI-X requested by a device in a

different property than for MSI. Pull the property name out as a
parameter and share the logic for both cases.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>