🔧 Update thermocouple 2 pin sanity check (MarlinFirmware#25627)

Author: ellensp

Marlin/src/inc/SanityCheck.h

@@ -2526,8 +2526,8 @@ static_assert(X_MAX_LENGTH >= X_BED_SIZE, "Movement bounds (X_MIN_POS, X_MAX_POS
       #error "TEMP_SENSOR_2 is required with 3 or more HOTENDS."
     #elif !HAS_HEATER_2
       #error "HEATER_2_PIN not defined for this board."
-    #elif !PIN_EXISTS(TEMP_2) && !TEMP_SENSOR_2_IS_DUMMY
-      #error "TEMP_2_PIN not defined for this board."
+    #elif !ANY_PIN(TEMP_2, TEMP_2_CS) && !TEMP_SENSOR_2_IS_DUMMY
+      #error "TEMP_2_PIN or TEMP_2_CS_PIN not defined for this board."
     #endif
     #if HOTENDS > 3
       #if TEMP_SENSOR_3 == 0

Marlin/src/pins/sam/pins_ULTRATRONICS_PRO.h

@@ -153,7 +153,7 @@
 #define TEMP_0_CS_PIN                         65
 #define TEMP_1_CS_PIN                         52
 #define TEMP_2_CS_PIN                         50
-#define TEMP_3_CS_PIN                         51
+#define TEMP_3_CS_PIN                         51  // Not yet supported
 
 #define ENC424_SS                             61