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: NMartin Fuzzey <martin.fuzzey@flowbird.group>
Reviewed-by: NFugang Duan <fugang.duan@nxp.com>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/fsl-fec.txt

@@ -22,6 +22,8 @@ Optional properties:
 - fsl,err006687-workaround-present: If present indicates that the system has
   the hardware workaround for ERR006687 applied and does not need a software
   workaround.
+- gpr: phandle of SoC general purpose register mode. Required for wake on LAN
+  on some SoCs
  -interrupt-names:  names of the interrupts listed in interrupts property in
   the same order. The defaults if not specified are
   __Number of interrupts__   __Default__