spapr_pci: fix device tree props for MSI/MSI-X

PAPR requires ibm,req#msi and ibm,req#msi-x to be present in the
device node to define the number of msi/msi-x interrupts the device
supports, respectively.

Currently we have ibm,req#msi-x hardcoded to a non-sensical constant
that happens to be 2, and are missing ibm,req#msi entirely. The result
of that is that msi-x capable devices get limited to 2 msi-x
interrupts (which can impact performance), and msi-only devices likely
wouldn't work at all. Additionally, if devices expect a minimum that
exceeds 2, the guest driver may fail to load entirely.

SLOF still owns the generation of these properties at boot-time
(although other device properties have since been offloaded to QEMU),
but for hotplugged devices we rely on the values generated by QEMU
and thus hit the limitations above.

Fix this by generating these properties in QEMU as expected by guests.

In the future it may make sense to modify SLOF to pass through these
values directly as we do with other props since we're duplicating SLOF
code.

Cc: qemu-ppc@nongnu.org
Cc: qemu-stable@nongnu.org
Cc: David Gibson <david@gibson.dropbear.id.au>
Cc: Nikunj A Dadhania <nikunj@linux.vnet.ibm.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
