dt-bindings: net: fec: convert fsl,*fec bindings to yaml

In order to automate the verification of DT nodes convert fsl-fec.txt to
fsl,fec.yaml, and pass binding check with below command.

$ make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- dt_binding_check DT_SCHEMA_FILES=Documentation/devicetree/bindings/net/fsl,fec.yaml
  DTEX    Documentation/devicetree/bindings/net/fsl,fec.example.dts
  DTC     Documentation/devicetree/bindings/net/fsl,fec.example.dt.yaml
  CHECK   Documentation/devicetree/bindings/net/fsl,fec.example.dt.yaml

Signed-off-by: Joakim Zhang <qiangqing.zhang@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>