soc/intel/elkhartlake: Hook up S0ix setting to option API

Hook up the s0ix_enable setting to the option API, so it can be changed
at runtime without recompilation. Default to the value set by the
mainboard.

Change-Id: I2d2f5c1587bd86c8fee634a49e1ec989c2bef783
Signed-off-by: Matt DeVillier <matt.devillier@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/87499
Reviewed-by: Jérémy Compostella <jeremy.compostella@intel.com>
Reviewed-by: Sean Rhodes <sean@starlabs.systems>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/soc/intel/elkhartlake/fsp_params.c

@@ -505,6 +505,9 @@ void platform_fsp_silicon_init_params_cb(FSPS_UPD *supd)
 
 	/* Override/Fill FSP Silicon Param for mainboard */
 	mainboard_silicon_init_params(params);
+
+	/* Runtime configuration of S0ix */
+	config->s0ix_enable = get_uint_option("s0ix_enable", config->s0ix_enable);
 }
 
 /* Mainboard GPIO Configuration */