Documentation: macb: Document phy-handle binding

Document the existence of the optional binding, directing to the
general ethernet document that describes this binding.

Signed-off-by: Brad Mouring <brad.mouring@ni.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>