net: ipv6: Tailor PE values for Zephyr

Making default preferred time much closer to default valid time
so that the served addresses are much shorter time in deprecated
state where they cannot be utilized. This is important in Zephyr
as there are limited number of IPv6 addresses that can be allocated
to the network interface.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>