rtc: tps6586x: remove mktime usage

The tps6586x use a 64-bit 'epoch_start' value, but then computes that
value using an 'mktime()', which has a smaller range and overflows
in 2106 at the latest. As both the hardware and the subsystem interface
support wider than 32-bit ranges for rtc times here, let's change all
the operations on 'seconds' to time64_t.

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