arm64: dts: juno: Fix "debounce-interval" property misspelling

"debounce_interval" was never supported in the bindings. It should be
"debounce-interval". Moreover, latest DTC complains the following:

Warning (property_name_chars_strict): debounce_interval: Character '_' not recommended in property name

This patch fixes the above warning by using the correct property as
per the bindings.
Acked-by: Liviu Dudau <liviu.dudau@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>

arch/arm64/boot/dts/arm/juno-motherboard.dtsi

@@ -58,42 +58,42 @@ gpio_keys {
 				compatible = "gpio-keys";
 
 				power-button {
-					debounce_interval = <50>;
+					debounce-interval = <50>;
 					wakeup-source;
 					linux,code = <116>;
 					label = "POWER";
 					gpios = <&iofpga_gpio0 0 0x4>;
 				};
 				home-button {
-					debounce_interval = <50>;
+					debounce-interval = <50>;
 					wakeup-source;
 					linux,code = <102>;
 					label = "HOME";
 					gpios = <&iofpga_gpio0 1 0x4>;
 				};
 				rlock-button {
-					debounce_interval = <50>;
+					debounce-interval = <50>;
 					wakeup-source;
 					linux,code = <152>;
 					label = "RLOCK";
 					gpios = <&iofpga_gpio0 2 0x4>;
 				};
 				vol-up-button {
-					debounce_interval = <50>;
+					debounce-interval = <50>;
 					wakeup-source;
 					linux,code = <115>;
 					label = "VOL+";
 					gpios = <&iofpga_gpio0 3 0x4>;
 				};
 				vol-down-button {
-					debounce_interval = <50>;
+					debounce-interval = <50>;
 					wakeup-source;
 					linux,code = <114>;
 					label = "VOL-";
 					gpios = <&iofpga_gpio0 4 0x4>;
 				};
 				nmi-button {
-					debounce_interval = <50>;
+					debounce-interval = <50>;
 					wakeup-source;
 					linux,code = <99>;
 					label = "NMI";