rtc: Introduce rtc_tm_sub() helper function

There're many sites need comparing the two rtc_time variants for many
rtc drivers, especially in the instances of rtc_class_ops::set_alarm().

So add this common helper function to make things easy.
Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Xunlei Pang <pang.xunlei@linaro.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>

include/linux/rtc.h

@@ -24,6 +24,14 @@ extern void rtc_time64_to_tm(time64_t time, struct rtc_time *tm);
 ktime_t rtc_tm_to_ktime(struct rtc_time tm);
 struct rtc_time rtc_ktime_to_tm(ktime_t kt);
 
+/*
+ * rtc_tm_sub - Return the difference in seconds.
+ */
+static inline time64_t rtc_tm_sub(struct rtc_time *lhs, struct rtc_time *rhs)
+{
+	return rtc_tm_to_time64(lhs) - rtc_tm_to_time64(rhs);
+}
+
 /**
  * Deprecated. Use rtc_time64_to_tm().
  */