gpio: board.txt: Fix the gpio name example

As explained in this file:

"GPIOs mappings are defined in the consumer device's node, in a property named
<function>-gpios"

So fix the example to match the convention.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>