power: ab8500_fg.c: use 64-bit time types

This patch changes 32-bit time types to 64-bit in
drivers/power/ab8500_fg.c

timespec and time_t can only represent signed 32-bit
dates but the driver should represent dates that are
after January 2038. So used time64.h header file and
its proper types and functions.

Use time64_t type instead of __kernel_time_t for
time_stamps variable of ab8500_fg_avg_cap struct

Signed-off-by: Ebru Akagunduz <ebru.akagunduz@gmail.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Sebastian Reichel <sre@kernel.org>