net: move devres helpers into a separate source file

There's currently only a single devres helper in net/ - devm variant
of alloc_etherdev. Let's move it to net/devres.c with the intention of
assing a second one: devm_register_netdev(). This new routine will need
to know the address of the release function of devm_alloc_etherdev() so
that it can verify (using devres_find()) that the struct net_device
that's being passed to it is also resource managed.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: David S. Miller <davem@davemloft.net>