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: 5fd974d164b4 ("drm/i915/mtl: add initial definitions for GSC CS")
Suggested-by: Matt Roper <[e-mail]>
Cc: Daniele Ceraolo Spurio <[e-mail]>
Cc: Tvrtko Ursulin <[e-mail]>
Signed-off-by: Rodrigo Vivi <[e-mail]>
Reviewed-by: Matt Roper <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

Documentation/gpu/i915.rst

···
   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