drm/xe/mcr: Document how to initialize group/instance

Add a sentence about the initialization so it's clear for newcomers how
to tweak the init functions for new platforms.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>