Enable the XUSB pad controller and XUSB controller (implementing an XHCI
interface) in the multi_v7 default configuration.

The XUSB pad controller is built-in because it is needed by PCIe, which
in turn provides the onboard ethernet used for network boot on a Jetson
TK1.
Signed-off-by: Thierry Reding <treding@nvidia.com>