arch-arm: Correction to address size in EL2/EL3

This commit corrects how the address size is
determined in EL2/EL3.  Previously, TCR_ELx.ips
was used but this should be TCR_ELx.ps.

Change-Id: I7e5a2f376335532a1d1c8c74d12a416617474ae2
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/12551
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>