[efi] Retain a long-lived reference to the EFI_PCI_IO_PROTOCOL instance

[efi] Retain a long-lived reference to the EFI_PCI_IO_PROTOCOL instance

Provide opened EFI PCI devices with access to the underlying
EFI_PCI_IO_PROTOCOL instance, in order to facilitate the future use of
the DMA mapping methods within the fast data path.

Do not require the use of this stored EFI_PCI_IO_PROTOCOL instance for
memory-mapped I/O (since the entire point of memory-mapped I/O as a
concept is to avoid this kind of unnecessary complexity) or for
slow-path PCI configuration space accesses (since these may be
required for access to PCI bus:dev.fn addresses that do not correspond
to a device bound via our driver binding protocol instance).

Signed-off-by: Michael Brown <mcb30@ipxe.org>