dt-bindings: net: snps,dwmac: add missing DWMAC IP version

Add missing DWMAC IP version in snps,dwmac.yaml which found by below
command, as NXP i.MX8 families support SNPS DWMAC 5.10a IP.

$ make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- dt_binding_check DT_SCHEMA_FILES=Documentation/devicetree/bindings/net/nxp,dwmac-imx.yaml
Documentation/devicetree/bindings/net/nxp,dwmac-imx.example.dt.yaml:
ethernet@30bf0000: compatible: None of ['nxp,imx8mp-dwmac-eqos', 'snps,dwmac-5.10a'] are valid under the given schema

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