hw/ppc/mac_newworld: Restrict RAM to 2 GiB

On Mac99 and newer machines, the Uninorth PCI host bridge maps
the PCI hole region at 2GiB, so the RAM area beside 2GiB is not
accessible by the CPU. Restrict the memory to 2GiB to avoid
problems such the one reported in the buglink.

Buglink: https://bugs.launchpad.net/qemu/+bug/1922391


Reported-by: Håvard Eidnes <he@NetBSD.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210406084842.2859664-1-f4bug@amsat.org>
Reviewed-by: BALATON Zoltan <balaton@eik.bme.hu>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>