dt-binding: net: wireless: fix node name in the BCM43xx example

The example in the BCM43xx documentation uses "brcmf" as node name.
However, wireless devices should be named "wifi" instead. Fix this to
make sure that .dts authors can simply use the documentation as
reference (or simply copy the node from the documentation and then
adjust only the board specific bits).

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Acked-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Rob Herring <robh@kernel.org>