powerpc/powernv: Make PCI non-optional

Bare metal systems without PCI don't exist, so there's no real point in
making PCI optional, it just breaks the build from time to time. In fact
the build is broken now if you turn off PCI_MSI but enable KVM.

Using select for PCI is OK because we (powerpc) define config PCI, and it
has no dependencies. Selecting PCI_MSI is slightly fishy, because it's
in drivers/pci and it is user-visible, but its only dependency is PCI,
so selecting it can't actually lead to breakage.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Acked-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>