x86/resctrl: Refactor in preparation for node-scoped resources

Sub-NUMA cluster systems provide monitoring resources at the NUMA
node scope instead of the L3 cache scope.

Rename the cache_level field in struct rdt_resource to the more
generic "scope" and add symbolic names and a helper function.

No functional change.

Signed-off-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: James Morse <james.morse@arm.com>
Reviewed-by: Peter Newman <peternewman@google.com>