Platform/RPi4: Add XHCI ACPI table

Since the RPi4 PCIe host bridge is not ECAM compliant, we can
not expose it as a host bridge to the OS via ACPI. However,
given the hardwired nature of this platform, we can expose the
xHCI controller that is guaranteed to live at the base of the
MMIO32 BAR window as a platform device directly.

It should be noted that the xHCI table is not finalized at this
stage, as Windows xHCI support is still a major question mark.

Signed-off-by: Pete Batard <pete@akeo.ie>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>