USB: xhci: Add Makefile, MAINTAINERS, and Kconfig entries.

Add Makefile and Kconfig entries for the xHCI host controller driver.
List Sarah Sharp as the maintainer for the xHCI driver.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>