USB support for RMI processors

[C. Jayachandran]

I've two patches to add USB EHCI support for XLS processors. This is tested
with umass driver on a USB attached disk and flash drive. Please review -
I've left the original NetBSD license on xls_ehci.c (which came from
ehci_pci.c), I hope this is not an issue.

http://sites.google.com/site/cjayachandran/files/iodi-bus.patch
- Move rmi_pci_bus_space to header and avoid extern
- remove unused and commented code (MIPS_BUS_SPACE_PCI, pic_usb_ack)
- use rmi_pci_bus_space for USB too (needs byteswap)

http://sites.google.com/site/cjayachandran/files/xls-ehci.patch
- uncomment xls_ehci.c in files.xlr
- changes to xls_ehci.c - updated with dev/usb/controller/ehci_*.c as
reference

The files sys/mips/rmi/ehcireg.h and sys/mips/rmi/ehcivar.h can be deleted
since they are not used now. Also sys/mips/rmi/pcibus.c is unused.

Thanks,
JC.