Platform/RaspberryPi: Add XHCI/PCI selection menu

Arm has standardized a PCI SMC conduit that can be used
to access the PCI config space in a standardized way. This
functionality doesn't yet exist in many OS/Distro's. Lets
add another advanced config item that allows the user
to toggle between presenting the XHCI on the base RPi4
as a platform device, or presenting this newer PCIe
conduit. The CM4 doesn't have an attached XHCI controller
soldered to the PCIe, so PCIe mode is the default.

Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
Reviewed-by: Andrei Warkentin <awarkentin@vmware.com>
Reviewed-By: Samer El-Haj-Mahmoud <Samer.El-Haj-Mahmoud@arm.com>
Tested-by: Jared McNeill <jmcneill@invisible.ca>