xen: remove XEN_PLATFORM_PCI config option

Xen PVHVM needs xen-platform-pci, on the other hand xen-platform-pci is
useless in any other cases.
Therefore remove the XEN_PLATFORM_PCI config option and compile
xen-platform-pci built-in if XEN_PVHVM is selected.

Changes to v1:

- remove xen-platform-pci.o and just use platform-pci.o since it is not
externally visible anymore.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>