cacheinfo: Add helper to access any cache index for a given CPU

The cacheinfo for a given CPU at a given index is used at quite a few
places by fetching the base point for index 0 using the helper
per_cpu_cacheinfo(cpu) and offsetting it by the required index.

Instead, add another helper to fetch the required pointer directly and
use it to simplify and improve readability.

Tested-by: Ionela Voinescu <ionela.voinescu@arm.com>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>