net/fec: gasket needs to be enabled for some i.mx

On the recent i.mx (mx25/50/53), there is a gasket inside fec
controller which needs to be enabled no matter phy works in MII
or RMII mode.

The current code enables the gasket only when phy interface is RMII.
It's broken when the driver works with a MII phy.  The patch uses
platform_device_id to distinguish the SoCs that have the gasket and
enables it on these SoCs for both MII and RMII mode.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Reported-by: Troy Kisky <troy.kisky@boundarydevices.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: David S. Miller <davem@davemloft.net>