xhci: Parse and print extended capabilities

Parse out and display xhci controller extended capabilities. This gives
us a mechanism to pick these up when the controller attaches. The print
helps users and developer know the availability of features in the wild.

This change will let developers know from dmesg that a controller
supports USB debugging.

Reviewed by: aokblast
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D51255