drm/i915/mtl: add initial definitions for GSC CS

Starting on MTL, the GSC is no longer managed with direct MMIO access,
but we instead have a dedicated command streamer for it. As a first step
for adding support for this CS, add the required definitions.
Note that, although it is now a CS, the GSC retains its old
class:instance value (OTHER_CLASS instance 6)

Bspec: 65308, 45605
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221102171047.2787951-2-daniele.ceraolospurio@intel.com