gem5/gem5-aarch64-native: update to v20.1.0.3

Update to the latest tagged version of gem5. Previously included
patches are now part of that release.
Upstream patch backported to enable boot with SMMUv3 model.

Change-Id: I7d7ad6f9681eb1f06743d214abbe901e9f8aa74e
Signed-off-by: Jon Mason <jon.mason@arm.com>
Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Adrian Herrera <adrian.herrera@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>