Prepare for an alternate GuC communication interface.

v2: Make a few functions static and name them correctly while we are at it (Oscar), but
leave an intel_guc_send_mmio interface for users that require old-style communication.

v3: Send intel_uc_init_early back to the top (Michal).
Signed-off-by: NMichel Thierry <michel.thierry@intel.com>
Signed-off-by: NMichal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: NOscar Mateo <oscar.mateo@intel.com>
Reviewed-by: NDaniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: NJoonas Lahtinen <joonas.lahtinen@linux.intel.com>