dt-bindings: w1-gpio: Remove unneeded unit address

Remove unneeded unit address from onewire node, so that dtc does not
complain that unit address is present without a corresponding reg entry,
when such use is made on a real dts file.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Rob Herring <robh@kernel.org>