Add a phy-num property to the i.MX FEC emulator

Add a phy-num property to the i.MX FEC emulator

We need a solution to use an Ethernet PHY that is not the first device
on the MDIO bus (device 0 on MDIO bus).

As an example with the i.MX6UL the NXP SOC has 2 Ethernet devices but
only one MDIO bus on which the 2 related PHY are connected but at unique
addresses.

Signed-off-by: Jean-Christophe Dubois <jcd@tribudubois.net>
Message-id: a1a5c0e139d1c763194b8020573dcb6025daeefa.1593296112.git.jcd@tribudubois.net
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>