drm/i915/uc: Introduce GSC FW

On MTL the GSC FW needs to be loaded on the media GT by the graphics
driver. We're going to treat it like a new uc_fw, so add the initial
defs and init/fini functions for it.

Similarly to the other FWs, the GSC FW path can be overridden via
modparam. The modparam can also be used to disable the GSC FW loading by
setting it to an empty string.

Note that the new structure has been called intel_gsc_uc to avoid
confusion with the existing intel_gsc, which instead represents the heci
gsc interfaces.

v2: re-order Makefile list to be properly sorted (Jani, Alan), better
    comment (alan)

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Alan Previn <alan.previn.teres.alexis@intel.com>
Cc: John Harrison <John.C.Harrison@Intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Reviewed-by: Alan Previn <alan.previn.teres.alexis@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221208200521.2928378-2-daniele.ceraolospurio@intel.com