ARM: shmobile: kzm9g-reference: Use keyboard as gpio-keys node name

This appears to be the best match for ePAPR.
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>

arch/arm/boot/dts/sh73a0-kzm9g-reference.dts

@@ -113,7 +113,7 @@ led4 {
 		};
 	};
 
-	gpio-keys {
+	keyboard {
 		compatible = "gpio-keys";
 
 		back-key {