hw/rtc/aspeed_rtc: Use 64-bit offset for holding time_t difference

In the aspeed_rtc device we store a difference between two time_t
values in an 'int'. This is not really correct when time_t could
be 64 bits. Enlarge the field to 'int64_t'.

This is a migration compatibility break for the aspeed boards.
While we are changing the vmstate, remove the accidental
duplicate of the offset field.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Cédric Le Goater <clg@kaod.org>