PCI: Move enum pci_interrupt_pin to linux/pci.h

We currently have a definition of enum pci_interrupt_pin in a header
specific to PCI endpoints - linux/pci-epf.h. In order to allow for use of
this enum from PCI host code in a future commit, move its definition to
linux/pci.h & include that from linux/pci-epf.h.

Additionally we add a PCI_NUM_INTX macro which indicates the number of PCI
INTx interrupts, and will be used alongside enum pci_interrupt_pin in
further patches.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
[bhelgaas: move enum pci_interrupt_pin outside #ifdef CONFIG_PCI]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>