rtc: Modify rtc_hctosys() to address y2038 issues

rtc_hctosys() has a number of y2038 issues.

This patch resolves them by:
- Replace rtc_tm_to_time() with y2038-safe rtc_tm_to_time64()
- Replace do_settimeofday() with y2038-safe do_settimeofday64()

After this patch, it should not have any remaining y2038 issues.

Cc: pang.xunlei <pang.xunlei@linaro.org>
Cc: Arnd Bergmann <arnd.bergmann@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Xunlei Pang <pang.xunlei@linaro.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>