arm64: dts: imx8qxp: add "fsl,imx8qm-fec" compatible string for FEC

Add "fsl,imx8qm-fec" compatible string for FEC to support new feature
(RGMII delayed clock).

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