[usb] Break out the USB controller and IO bus parts into separate modules

[adrian]

Summary

This is a work in progress for breaking out the USB controller and
its IO bus into separate pieces, rather than having the USB module
include PCI code.

This allows for USB to be loadable as a module on things that don't
sit USB on a PCI bus, such as most embedded ARM, PPC and MIPS hardware.

Test Plan

I've tested this on the Atheros MIPS boards with a module for the ohci
and ehci controllers. I'll dig those module Makefiles up and add it
to this patch.

[• hselasky]

sys/modules/usb/uhci_pci/Makefile

32

uhci_pci ?

sys/modules/usb/xhci_pci/Makefile

32

xhci_pci ?