eth: dm: fec: Change FEC PHY mask setting from CONFIG_PHYLIB to CONFIG_FEC_MXC_PHYADDR

Without this commit we do have an explicit dependency on CONFIG_PHYLIB
when one wants to set PHY ADDR on a iMX board (FEC + driver model).

This shall be changed to CONFIG_FEC_MXC_PHYADDR, as only when we do have
it set, we shall mask out other devices.

As a side effect, when CONFIG_FEC_MXC_PHYADDR is not set, we scan PHY bus
for connected PHY devices.

Signed-off-by: Lukasz Majewski <lukma@denx.de>