dt-bindings: fec: document the new gpr property.

This property allows the gpr register bit to be defined
for wake on lan support.

Signed-off-by: Martin Fuzzey <martin.fuzzey@flowbird.group>
Reviewed-by: Fugang Duan <fugang.duan@nxp.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>