dt-bindings: net: imx-dwmac: Add NXP imx8 DWMAC glue layer

Add description for NXP imx8 families like imx8mp/imx8dxl
that integrate the Synopsys gmac IP version 5.10a.

Signed-off-by: Fugang Duan <fugang.duan@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>