Platform/RaspberryPi/Drivers/PlatformSmbiosDxe: improve UX with 3GB limit

Right now there was no way to tell you're booting with RAM limited
to 3GB, since the setup front page still listed 4096 MB.

Fix this by honoring PcdRamLimitTo3GB in PlatformSmbiosDxe.

Tested on 2GB and 4GB boards (with limiting and without)

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