New component to support Raspberry Pi 4
Anticipating XHCIDriver being added to the Components file for BC2835 at some point, add it to the superproject.
Note, this doesn't imply that the specified revision of XHCIDriver will necessarily support Pi 4; the aim of this change is to start getting the remote tracking branch version of this component into the nightly build tree.
PCIManager is already in the superproject. Have I missed any other existing components that will need to be added?