KVM: arm64: selftests: Data type cleanup for arch_timer test

Change signed type to unsigned in test_args struct which
only make sense for unsigned value.

Suggested-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Haibo Xu <haibo1.xu@intel.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Anup Patel <anup@brainfault.org>