Platform/RaspberryPi: Add Negative table check

Turns out its helpful to have a !PcdToken flag
that enables a DSDT/SSDT. That simplifies
both the emmc2 SSDT (it only installs when
!SdIsArasan) and later for the XHCI/PCIe switch
where we want to install one of two tables
depending on whether a single Pcd is set.

Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
Reviewed-by: Pete Batard <pete@akeo.ie>
Reviewed-by: Andrei Warkentin <awarkentin@vmware.com>