Platform/RaspberryPi: Only enable IORT when 3G limit is disabled.

The 3G limit, and the 2G IORT are intended to solve
the same linux problem. They limit PCI DMA operations
to the first 3G of RAM. Older linux kernels, as
used with RHEL/Centos, trigger an assertion*
when a DMA operation starts at a range that
doesn't fit within the 2G range specified by the IORT.

The simple solution is to only enable the IORT
when the 3G flag is disabled and there is more
than 3G installed.

* https://github.com/pftf/RPi4/issues/123



Fixes: dac891da ("Platform/RaspberryPi/AcpiTables: add a IORT ACPI table to limit XHCI DMA")
Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
Reviewed-by: Andrei Warkentin <awarkentin@vmware.com>