Fix SAMD51 timer usage (#17471)

Marlin/src/HAL/SAMD51/inc/SanityCheck.h

@@ -35,6 +35,10 @@
   #error "OnBoard SPI BUS can't be shared with other devices."
 #endif
 
+#if SERVO_TC == RTC_TIMER_NUM
+  #error "Servos can't use RTC timer"
+#endif
+
 #if ENABLED(EMERGENCY_PARSER)
   #error "EMERGENCY_PARSER is not yet implemented for SAMD51. Disable EMERGENCY_PARSER to continue."
 #endif