Add FEC support for Engicam i.CoreM6 dql modules.

Observed similar 'eth0: link is not ready' issue which was
discussed in [1] due rmii mode with external ref_clk, so added
clock node along with the properties mentioned by Shawn in [2]

FEC link log:
------------
$ ifconfig eth0 up
[   27.905187] SMSC LAN8710/LAN8720 2188000.ethernet:00: attached PHY driver
               [SMSC LAN8710/LAN8720] (mii_bus:phy_addr=2188000.ethernet:00, irq=-1)
[   27.918982] IPv6: ADDRCONF(NETDEV_UP): eth0: link is not ready

[1] https://patchwork.kernel.org/patch/3491061/
[2] https://patchwork.kernel.org/patch/3490511/

Cc: Sascha Hauer <kernel@pengutronix.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Matteo Lisi <matteo.lisi@engicam.com>
Cc: Michael Trimarchi <michael@amarulasolutions.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>