hw/rtc/twl92230: Use int64_t for sec_offset and alm_sec

In the twl92230 device, use int64_t for the two state fields
sec_offset and alm_sec, because we set these to values that
are either time_t or differences between two time_t values.

These fields aren't saved in vmstate anywhere, so we can
safely widen them.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>