net: rpl: Allow fine tuning of parent count

Instead of always have CONFIG_NET_IPV6_MAX_NEIGHBORS parents,
use CONFIG_NET_RPL_MAX_PARENTS when creating the parent table.
Default value for max parents is the max neighbors so no
functionality changes are introduced here.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>