arm64: dts: imx8mp-beacon: remove fallback ethernet-phy-ieee802.3-c22

arm64: dts: imx8mp-beacon: remove fallback ethernet-phy-ieee802.3-c22

Remove the fallback compatible string "ethernet-phy-ieee802.3-c22" from the
Ethernet PHY node to fix below CHECK_DTB warning:

arch/arm64/boot/dts/freescale/imx8mp-beacon-kit.dtb: ethernet-phy@3 (ethernet-phy-id0022.1640): compatible: ['ethernet-phy-id0022.1640', 'ethernet-phy-ieee802.3-c22'] is too long
        from schema $id: http://devicetree.org/schemas/net/micrel,gigabit.yaml

Signed-off-by: Frank Li <Frank.Li@nxp.com>