dt-bindings: net: dwmac: document 'mac-mode' property

This change documents the 'mac-mode' property that was introduced in the
'stmmac' driver to support passive mode converters that can sit in-between
the MAC & PHY.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: David S. Miller <davem@davemloft.net>