Support two MAX6675 thermocouples (#8686)

Marlin/src/module/temperature.h

@@ -632,8 +632,23 @@ class Temperature {
 
     static void updateTemperaturesFromRawValues();
 
-    #if ENABLED(HEATER_0_USES_MAX6675)
-      static int read_max6675();
+    #define HAS_MAX6675 (ENABLED(HEATER_0_USES_MAX6675) || ENABLED(HEATER_1_USES_MAX6675))
+    #if HAS_MAX6675
+      #if ENABLED(HEATER_0_USES_MAX6675) && ENABLED(HEATER_1_USES_MAX6675)
+        #define COUNT_6675 2
+      #else
+        #define COUNT_6675 1
+      #endif
+      #if COUNT_6675 > 1
+        #define READ_MAX6675(N) read_max6675(N)
+      #else
+        #define READ_MAX6675(N) read_max6675()
+      #endif
+      static int read_max6675(
+        #if COUNT_6675 > 1
+          const uint8_t hindex=0
+        #endif
+      );
     #endif
 
     static void checkExtruderAutoFans();