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: NJulien 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: NAndreas Färber <afaerber@suse.de>
Reviewed-by: NJavier Martinez Canillas <javier@osg.samsung.com>
Reviewed-by: NJulien Chauveau <chauveau.julien@gmail.com>
Signed-off-by: NRob Herring <robh@kernel.org>

Documentation/devicetree/bindings/input/gpio-keys.txt

@@ -32,17 +32,17 @@ Optional subnode-properties:
 
 Example nodes:
 
-	gpio_keys {
+	gpio-keys {
 			compatible = "gpio-keys";
-			#address-cells = <1>;
-			#size-cells = <0>;
 			autorepeat;
-			button@21 {
+
+			up {
 				label = "GPIO Key UP";
 				linux,code = <103>;
 				gpios = <&gpio1 0 1>;
 			};
-			button@22 {
+
+			down {
 				label = "GPIO Key DOWN";
 				linux,code = <108>;
 				interrupts = <1 IRQ_TYPE_LEVEL_HIGH 7>;