ARM: dts: stm32: Remove extra size-cells on dhcom-pdk2

Fix make dtbs_check warning:
arch/arm/boot/dts/stm32mp157c-dhcom-pdk2.dt.yaml: gpio-keys-polled: '#address-cells' is a dependency of '#size-cells'
arch/arm/boot/dts/stm32mp157c-dhcom-pdk2.dt.yaml: gpio-keys: '#address-cells' is a dependency of '#size-cells'
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Alexandre Torgue <alexandre.torgue@st.com>
Cc: Patrice Chotard <patrice.chotard@st.com>
Cc: Patrick Delaunay <patrick.delaunay@st.com>
Cc: linux-stm32@st-md-mailman.stormreply.com
To: linux-arm-kernel@lists.infradead.org
Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>

arch/arm/boot/dts/stm32mp15xx-dhcom-pdk2.dtsi

@@ -34,7 +34,6 @@ display_bl: display-bl {
 
 	gpio-keys-polled {
 		compatible = "gpio-keys-polled";
-		#size-cells = <0>;
 		poll-interval = <20>;
 
 		/*
@@ -60,7 +59,6 @@ button-2 {
 
 	gpio-keys {
 		compatible = "gpio-keys";
-		#size-cells = <0>;
 
 		button-1 {
 			label = "TA2-GPIO-B";