xhci: plat: adapt to unified device property interface

Requesting the only property that the driver needs using the
unified device property interface so it will be available
for all types of platforms, not just the ones using DT.

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>