sam460ex: Fix support for memory larger than 1GB

sam460ex: Fix support for memory larger than 1GB

Fix the encoding of larger memory modules in the SoC registers which
allows specifying more than 1GB memory for sam460ex. Well, only 2GB
due to SoC and firmware restrictions which was the only missing value
compared to what the real hardware supports. The SoC should support up
to 4GB but when setting that the firmware hangs during memory test.
This may be an overflow bug in the firmware which I did not try to
debug but this may affect real hardware as well.

Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>