arch-arm: Introduce ARMv8.1 Virtual Timer System Registers

arch-arm: Introduce ARMv8.1 Virtual Timer System Registers

Adding CNTHV_CTL_EL2, CNTHV_CVAL_EL2, CNTHV_TVAL_EL2 System Registers
into the decode tree. They are currently implemented as a generic timer
and produces a warning if accessed.

Change-Id: I1a23035d67f95eeac49d890283e9a0d58426d504
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/11592
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>