ARM: dts: imx6qdl-icore: Add fec phy-handle

LAN8720 needs a reset of every clock enable. The reset needs
to be done at device level, due the flag PHY_RST_AFTER_CLK_EN.

So, add phy-handle by creating mdio child node inside fec.
This will eventually move the phy-reset-gpio which is defined
in fec node.

Signed-off-by: Michael Trimarchi <michael@amarulasolutions.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>