rtc: ftrtc010: handle dates after 2106

Use correct types for offset and time and use
rtc_time64_to_tm/rtc_tm_to_time64 to handle dates after 2106 properly.

Acked-by: Hans Ulli Kroll <ulli.kroll@googlemail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>