net: fec: get regulator optional

According to the device tree binding the phy-supply property is
optional. Use the regulator_get_optional API accordingly. The
code already handles NULL just fine.

This gets rid of the following warning:
  fec 2188000.ethernet: 2188000.ethernet supply phy not found, using dummy regulator

Signed-off-by: Stefan Agner <stefan@agner.ch>
Reviewed-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Acked-by: Fugang Duan <fugang.duan@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>