boards: st: fix STM3MP1* boards DTS files coding style

Fix board DTS coding style issues to prevent they spread when a	new
board is introduced and used this DTS as example start point.

Issues addressed from Zephyr devicetree style guidelines [1]:
- Insert a single empty line to separate nodes at the same hierarchy level.

No functional change.

Link: https://docs.zephyrproject.org/latest/contribute/style/devicetree.html [1]
Signed-off-by: Etienne Carriere <etienne.carriere@st.com>

boards/st/stm32mp135f_dk/stm32mp135f_dk.dts

@@ -147,6 +147,7 @@
 	width = <480>;
 	height = <272>;
 	pixel-format = <PANEL_PIXEL_FORMAT_RGB_565>;
+
 	display-timings {
 		compatible = "zephyr,panel-timing";
 		de-active = <0>;
@@ -160,6 +161,7 @@
 		hfront-porch = <32>;
 		vfront-porch = <2>;
 	};
+
 	def-back-color-red = <0xFF>;
 	def-back-color-green = <0xFF>;
 	def-back-color-blue = <0xFF>;

boards/st/stm32mp157c_dk2/stm32mp157c_dk2.dts

@@ -29,6 +29,7 @@
 
 	leds {
 		compatible = "gpio-leds";
+
 		red_led_1: led_1 {
 			gpios = <&gpioh 7 GPIO_ACTIVE_HIGH>;
 			label = "LD7";
@@ -37,6 +38,7 @@
 
 	gpio_keys {
 		compatible = "gpio-keys";
+
 		user_button: button {
 			label = "User 1";
 			gpios = <&gpioa 14 GPIO_ACTIVE_LOW>;