dev-arm: Make CNTFRQ a GenericTimer parameter

dev-arm: Make CNTFRQ a GenericTimer parameter

This register should be in theory initialized by the highest
priviledged software. We do this in gem5 to avoid KVM
complications (the gem5 firmware won't run at highest EL)

JIRA: https://gem5.atlassian.net/browse/GEM5-611

Change-Id: I62d368105af48584f2fe9671de7c70b484b40c12
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Ciro Santilli <ciro.santilli@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/29612
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: Bobby R. Bruce <bbruce@ucdavis.edu>