drm/i915: Add missing GSCCS documentation

Introduce the basic documentation about GSC CS.

This "GPU Basics" section is focused on explaining the hardware
rather than the driver/uapi, so let's make sure GSC is also
properly documented here.

v2: Fixes from Matt: typos and acronym.

Fixes: 5fd974d164 ("drm/i915/mtl: add initial definitions for GSC CS")
Suggested-by: Matt Roper <matthew.d.roper@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230926175554.25968-1-rodrigo.vivi@intel.com

Documentation/gpu/i915.rst

@@ -279,6 +279,10 @@ An Intel GPU has multiple engines. There are several engine types:
   Also sometimes called 'VEBOX' in hardware documentation.
 - Compute Command Streamer (CCS). An engine that has access to the media and
   GPGPU pipelines, but not the 3D pipeline.
+- Graphics Security Controller (GSCCS). A dedicated engine for internal
+  communication with GSC controller on security related tasks like
+  High-bandwidth Digital Content Protection (HDCP), Protected Xe Path (PXP),
+  and HuC firmware authentication.
 
 The Intel GPU family is a family of integrated GPU's using Unified
 Memory Access. For having the GPU "do work", user space will feed the