dt-binding: ptp: fsl,ptp: add pci1957,ee02 compatible string for fsl,enetc-ptp

fsl,enetc-ptp is embedded pcie device. Add compatible string pci1957,ee02.

Fix warning:
arch/arm64/boot/dts/freescale/fsl-ls1028a-kontron-kbox-a-230-ls.dtb: ethernet@0,4:
	compatible:0: 'pci1957,ee02' is not one of ['fsl,etsec-ptp', 'fsl,fman-ptp-timer', 'fsl,dpaa2-ptp', 'fsl,enetc-ptp']

Signed-off-by: Frank Li <Frank.Li@nxp.com>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>