Input: gpio-keys-polled - don't use unit-address with button nodes

The change corrects an example in device tree documentation section,
because button/key device nodes don't contain reg property there is
no need to declare unit-address, and address and size cell properties
are also redundant.

At the moment a compilation of the hypothetical original example
should produce W=1 level warnings, and it is better to stop spreading
misusage of the polled gpio keys device tree binding through this
example.
Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

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

@@ -34,11 +34,10 @@ Example nodes:
 
 	gpio_keys_polled {
 			compatible = "gpio-keys-polled";
-			#address-cells = <1>;
-			#size-cells = <0>;
 			poll-interval = <100>;
 			autorepeat;
-			button@21 {
+
+			button21 {
 				label = "GPIO Key UP";
 				linux,code = <103>;
 				gpios = <&gpio1 0 1>;