dt-bindings: net: imx-dwmac: convert imx-dwmac bindings to yaml

In order to automate the verification of DT nodes covert imx-dwmac to
nxp,dwmac-imx.yaml, and pass below checking.

$ make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- dt_binding_check DT_SCHEMA_FILES=Documentation/devicetree/bindings/net/nxp,dwmac-imx.yaml
$ make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- dtbs_check DT_SCHEMA_FILES=Documentation/devicetree/bindings/net/nxp,dwmac-imx.yaml

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