cris: time: Cleanup of persistent clock stuff

- Remove update_persistent_clock(), as it does nothing now.
- Remove read_persistent_clock(), let it fall back to the weak version.

Cc: Mikael Starvik <starvik@axis.com>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Xunlei Pang <pang.xunlei@linaro.org>
Acked-by: Rabin Vincent <rabin@rab.in>
Signed-off-by: Jesper Nilsson <jespern@axis.com>