Add static inline rtcin_locked() and rtcout_locked() functions for doing a

Add static inline rtcin_locked() and rtcout_locked() functions for doing a
related series of operations without doing a lock/unlock for each byte.
Use them when reading and writing the entire set of time registers.

The original rtcin() and writertc() functions which do lock/unlock on each
byte still exist, because they are public and called by outside code.