dev-arm: Fix SMMUv3 ContextDescriptor pointer shift

dev-arm: Fix SMMUv3 ContextDescriptor pointer shift

The context descriptor pointer in the STE starts at the sixth LSB

Change-Id: Ifa346b350785b788e9d1e093b662cb26433adfb8
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Christo Smallwood <christo.smallwood@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/19469
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>