net: documentation: add docs for MAC/PHY support in DPAA2

Add documentation file for the MAC/PHY support in the DPAA2
architecture. This describes the architecture and implementation of the
interface between phylink and a DPAA2 network driver.

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>