arch-arm: Fix page size handling when merging stage 1 and 2

arch-arm: Fix page size handling when merging stage 1 and 2

The current code to merge translation entries from stage 1 and stage 2
doesn't handle cases where the page sizes at the different stages
differ. This change fixes both the case when the hypervisor has a
larger page size and when it has a smaller page size.

Change-Id: Icdf289005bf1e4de4d91d54643924a38d9d77796
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/10505
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>