Provide a default MAX6675_SS_PIN for ReARM

See #8010

Marlin/src/pins/pins_RAMPS_RE_ARM.h

@@ -196,6 +196,10 @@
 
 #define PS_ON_PIN          P2_12   // (12)
 
+#ifndef MAX6675_SS_PIN && !USE_ZMAX_PLUG
+  #define MAX6675_SS_PIN   P1_28
+#endif
+
 #if ENABLED(CASE_LIGHT_ENABLE) && !PIN_EXISTS(CASE_LIGHT) && !defined(SPINDLE_LASER_ENABLE_PIN)
   #if !defined(NUM_SERVOS) || NUM_SERVOS < 4   // Try to use servo connector
     #define CASE_LIGHT_PIN P1_18   // (4) MUST BE HARDWARE PWM