ARM: dts: stm32: fix joystick node on stm32f746 and stm32mp157c eval boards

"#size-cells" entry is not needed for "gpio-keys" driver. Indeed "reg"
entry is not used. This commit will fix a warnings seen by DT validation
tool.
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>

arch/arm/boot/dts/stm32746g-eval.dts

@@ -95,7 +95,6 @@ button@0 {
 
 	joystick {
 		compatible = "gpio-keys";
-		#size-cells = <0>;
 		pinctrl-0 = <&joystick_pins>;
 		pinctrl-names = "default";
 		button-0 {

arch/arm/boot/dts/stm32mp157c-ev1.dts

@@ -32,7 +32,6 @@ clk_ext_camera: clk-ext-camera {
 
 	joystick {
 		compatible = "gpio-keys";
-		#size-cells = <0>;
 		pinctrl-0 = <&joystick_pins>;
 		pinctrl-names = "default";
 		button-0 {