drm/i915/guc: Add documentation for MMIO based communication

As we are going to extend our use of MMIO based communication,
try to explain its mechanics and update corresponding definitions.

v2: fix checkpatch MACRO_ARG_REUSE

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Cc: Kelvin Gardiner <kelvin.gardiner@intel.com>
Reviewed-by: Michel Thierry <michel.thierry@intel.com> #1
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180326194829.58836-2-michal.wajdeczko@intel.com