Add millis helper macros

Marlin/src/core/millis_t.h

@@ -25,5 +25,9 @@
 
 typedef uint32_t millis_t;
 
+#define SEC_TO_MS(N) millis_t((N)*1000UL)
+#define MIN_TO_MS(N) SEC_TO_MS((N)*60UL)
+#define MS_TO_SEC(N) millis_t((N)/1000UL)
+
 #define PENDING(NOW,SOON) ((int32_t)(NOW-(SOON))<0)
 #define ELAPSED(NOW,SOON) (!PENDING(NOW,SOON))