net: ethernet: cpsw: Search childs for slave nodes

The current implementation searches the whole DT for nodes named
"slave".

This patch changes it to search only child nodes for slaves.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Acked-by: Mugunthan V N <mugunthanvnm@ti.com>
Acked-by: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>