powerpc: Set CONFIG_NET=y in defconfigs

Commit 5d6be6a5 ("scsi_netlink : Make SCSI_NETLINK dependent on NET
instead of selecting NET") removed what happened to be the only instance
of 'select NET'. Defconfigs that were relying on the select now lack
networking support.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: linuxppc-dev@lists.ozlabs.org
Signed-off-by: Michal Marek <mmarek@suse.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>