dt-bindings: net: add the DPAA2 MAC DTS definition

Add a documentation entry for the DTS bindings needed and supported by
the dpaa2-mac driver.

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>