rtc: ls1x: remove mktime usage

The loongson1 platform is 32-bit, so storing a time value in 32 bits
suffers from limited range. In this case it is likely to be correct
until 2106, but it's better to avoid the limitation and just use
the time64_t based mktime64() and rtc_time64_to_tm() interfaces.

The hardware uses a 32-bit year number, and time64_t can cover that
entire range.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>