rtc: sysfs: Use time64_t variables to set time/alarm

Use time64_t variables and related APIs for sysfs interfaces to
support setting time or alarm after the year 2038 on 32-bit system.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>