drm/xe/gsc: Introduce GSC FW

Add the basic definitions and init function. Same as HuC, GSC is only
supported on the media GT on MTL and newer platforms.
Note that the GSC requires submission resources which can't be allocated
during init (because we don't have the hwconfig yet), so it can't be
marked as loadable at the end of the init function. The allocation of
those resources will come in the patch that makes use of them to load
the FW.

v2: better comment, move num FWs define inside the enum (John)

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Alan Previn <alan.previn.teres.alexis@intel.com>
Cc: John Harrison <John.C.Harrison@Intel.com>
Reviewed-by: John Harrison <John.C.Harrison@Intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>