Some firmware does not require run-time mapping of the legacy IO port
space.  (It may not need to perform any IO port operations, or it may
do them with translation disabled.)

(efi_get_iobase): Don't require that IO port space be marked RT, since
there's no reason the firmware should require mappings for it.
Thanks to Greg Albrecht for noticing this.

Also, allow attributes in addition to EFI_MEMORY_UC.  I can't
think of another current attribute that makes sense, but the
kernel only depends on being able to use UC.