system-arm: AArch64 boot, init CNTFRQ_EL0

system-arm: AArch64 boot, init CNTFRQ_EL0

CNTFRQ_EL0 should be initialised to a uniform value in all cores present
in the system. Previously, this was only done if EL3 was present,
however architecture states CNTFRQ_EL0 may be written from the highest
EL implemented.
This patch moves this initilization outside of the EL3-only one.

Change-Id: Ibaa197de53d531ba898e5137ba4f46a8c9554699
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/24683
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>