Platform/RaspberryPi: Add Linux quirk support

Linux, for the time being has refused to support the Arm
standard SMCCC for PCIe configuration. Instead they
want to continue to maintain per device "quirks".

As the RPI isn't really ECAM this is a bit more
involved because the MCFG can't really describe
the root port+config registers situation. Further
platforms which support the SMCCC shouldn't have
a MCFG, so we need an additional way to tell linux
what it needs to know about this platform.

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>