PCI: add generic pcibios_resource_to_bus()

This replaces the generic versions of pcibios_resource_to_bus() and
pcibios_bus_to_resource() in asm-generic/pci.h with versions that use
pci_resource_to_bus() and pci_bus_to_resource().

The replacements are equivalent except that they can apply host
bridge window offsets when the arch has supplied them by using
pci_add_resource_offset().

Each arch can convert to using pci_add_resource_offset() individually by
removing its device resource fixups from pcibios_fixup_bus() and supplying
ARCH_HAS_GENERIC_PCI_OFFSETS.  ARCH_HAS_GENERIC_PCI_OFFSETS can be removed
after all have converted.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>