armv8: Make COUNTER_FREQUENCY optional

Some platforms have the means to determine the counter frequency at
runtime, so give them an opportunity to do so.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Tested-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>