Documentation: devicetree: Clean up gpio-keys example

Drop #address-cells and #size-cells, which are not required by the
gpio-keys binding documentation, as button sub-nodes are not devices.

Rename sub-nodes to avoid new dtc unit address warnings when copied.

While at it, adopt the dashes convention for the node name.

Reported-by: Julien Chauveau <chauveau.julien@gmail.com>
Cc: Julien Chauveau <chauveau.julien@gmail.com>
Cc: Javier Martinez Canillas <javier@dowhile0.org>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>
Reviewed-by: Julien Chauveau <chauveau.julien@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>