powerpc/pseries: Make PCI non-optional

The pseries build with PCI=n looks to have been broken for at least 5
years, and no one's noticed or cared.

Following the obvious breakages backward, the first commit I can find
that builds is the parent of 2eb4afb6

 ("powerpc/pci: Move pseries
code into pseries platform specific area") from April 2009.

A distro would never ship a PCI=n kernel, so it is only useful for folks
building custom kernels. Also on KVM the virtio devices appear on PCI,
so it would only be useful if you were building kernels specifically to
run on PowerVM and with no PCI devices.

The added code complexity, and testing load (which we've clearly not
been doing), is not justified by the small reduction in kernel size for
such a niche use case.

So just make PCI non-optional on pseries.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>