added si5338 spread spectrum setup

conf/machine/boards/elphel393/elphel393.dts

@@ -194,6 +194,7 @@
 				/* Disabled state for outputs:  */
 				si5338,dis_hi-z=    <0 1 2 3>; /* Disabled state for listed outputs, also possible: "dis_hi-z","dis_low","dis_high","dis_always_on" */
 				si5338,output_en=   < 3>; /* Which outputs should be initially enabled */
+				si5338,spread_spectrum_3= <1 50 31500>; /* Set spread spectrum for channel3 : enabled, 0.5%, 31.5KHz */
 			};
 			gpio@20{
 				compatible = "ti,tca6408";