net: ethernet: ti: cpsw: Assign OF node to slave devices

Assign OF node to CPSW slave devices, otherwise it is not possible to
bind e.g. DSA switch to them. Without this patch, the DSA code tries
to find the ethernet device by OF match, but fails to do so because
the slave device has NULL OF node.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: David S. Miller <davem@davemloft.net>
Cc: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>