Platform/RPi/AcpiTables: Fix ASL compilation warnings

Silence 2 warnings from pep.asl:
* "Warning 3133 - Unknown reserved name (_GPI/_GCI/_GDI)"
* "Warning 3150 - Empty Resource Template", which is caused by
   section "Name (_CRS, ResourceTemplate () {})"

Remove "Offset(0)" in Xhci.asl, which produces the warning:
"Unnecessary/redundant use of Offset operator".

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