net: macb: make magic-packet property generic

As requested by Rob Herring on patch
https://patchwork.ozlabs.org/patch/580862/

.

This is a new property that it's still in net-next and has never been
used in production, so we are not breaking anything with the
incompatible binding change.

Signed-off-by: Sergio Prado <sergio.prado@e-labworks.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>