rtc: vr41xx: remove mktime usage

This driver uses mktime() and rtc_time_to_tm() to convert between time
values. This works fine on 64-bit kernels over the whole supported
range, and the vr41xx chip is a 64-bit MIPS implementation, but it is
inconsistent because it doesn't do the same thing on 32-bit kernels that
overflow in 2106 or 2038.

Changing it to use mktime64/rtc_time64_to_tm() should have no visible
impact on vr41xx but gets us closer to removing the 32-bit interfaces.

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