drm/xe/bmg: Add BMG platform definition

BMG is a discrete GPU based on the Xe2 architecture.
No device ids are bound to the BMG platform descriptor yet.
BMG device ids will be added once we have all the basic required
platform enabling patches landed.

v2: Removed device ids, deferring it to a later patch

v3: Squash in compat header IS_BATTLEMAGE() patch.  (Lucas)

Bspec: 68090
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Balasubramani Vivekanandan <balasubramani.vivekanandan@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240408170545.3769566-4-balasubramani.vivekanandan@intel.com