rtc: sun4v: set range

The Sun4v Hypervisor Core API Specification states:
Time is described by a single unsigned 64-bit word equivalent to a time_t
for the POSIX time(2) system call. The word contains the time since the
Epoch (00:00:00 UTC, January 1, 1970), measured in seconds.

Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>