drm/i915/guc: Add Broxton GuC firmware loading support

Issue: VIZ-7772
Signed-off-by: Nick Hoath <nicholas.hoath@intel.com>
Reviewed-by:  <peter.antoine@intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1462531373-34819-2-git-send-email-nicholas.hoath@intel.com