[MIPS] Switch from to_tm to rtc_time_to_tm

This replaces the MIPS-specific to_tm function with the generic
rtc_time_to_tm function.  The big difference between the two functions is
that rtc_time_to_tm uses epoch 70 while to_tm uses 1970, so the result of
rtc_time_to_tm needs to be fixed up.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

include/asm-mips/time.h

@@ -48,13 +48,6 @@ extern void (*mips_timer_ack)(void);
  */
 extern struct clocksource clocksource_mips;
 
-/*
- * to_tm() converts system time back to (year, mon, day, hour, min, sec).
- * It is intended to help implement rtc_set_time() functions.
- * Copied from PPC implementation.
- */
-extern void to_tm(unsigned long tim, struct rtc_time *tm);
-
 /*
  * high-level timer interrupt routines.
  */