drm/xe/pmu: Add attribute skeleton

Add the generic support for defining new attributes. This only adds
the macros and common infra for the event counters, but no counters
yet. This is going to be added as follow up changes.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250124050411.2189060-5-lucas.demarchi@intel.com


Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>