drm/i915: Add GuC-related header files

intel_guc_fwif.h contains the subset of the GuC interface that we
will need for submission of commands through the GuC. These MUST
be kept in sync with the definitions used by the GuC firmware, and
updates to this file will (or should) be autogenerated from the
source files used to build the firmware. Editing this file is
therefore not recommended.

i915_guc_reg.h contains definitions of GuC-related hardware:
registers, bitmasks, etc. These should match the BSpec.

v2:
    Files renamed & resliced per review comments by Chris Wilson

v4:
    Added DON'T-EDIT-ME warning [Tom O'Rourke]

Issue: VIZ-4884
Signed-off-by: Alex Dai <yu.dai@intel.com>
Signed-off-by: Dave Gordon <david.s.gordon@intel.com>
Reviewed-by: Tom O'Rourke <Tom.O'Rourke@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>