arch-arm: Enable PMSELR_EL0 read in PMU

Change-Id: Ic1ec1d03464a7ce42295cee8a9f4ebcd3e90def1
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/17948
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>