EHCI enabled in GENERIC

[Ian Dowse]

The EHCI driver (USB 2.0) has been enabled in the -CURRENT GENERIC
config for the i386 and amd64 platforms. Let me know if this causes
problems. There are still some outstanding issues, but there are
also systems where USB does not function at all without EHCI enabled.

If there any severe problems then I'll remove it, or at least add
a tunable or hint to disable ehci.

Ian