boards: st: fix STM32F1 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/nucleo_f103rb/nucleo_f103rb.dts

@@ -24,6 +24,7 @@
 
 	leds: leds {
 		compatible = "gpio-leds";
+
 		green_led_2: led_2 {
 			gpios = <&gpioa 5 GPIO_ACTIVE_HIGH>;
 			label = "User LD2";
@@ -32,6 +33,7 @@
 
 	gpio_keys {
 		compatible = "gpio-keys";
+
 		user_button: button {
 			label = "User";
 			gpios = <&gpioc 13 GPIO_ACTIVE_LOW>;

boards/st/stm3210c_eval/stm3210c_eval.dts

@@ -22,6 +22,7 @@
 
 	leds {
 		compatible = "gpio-leds";
+
 		green_led_2: led_2 {
 			gpios = <&gpiod 13 GPIO_ACTIVE_HIGH>;
 			label = "User LD2";
@@ -30,6 +31,7 @@
 
 	gpio_keys {
 		compatible = "gpio-keys";
+
 		user_button: button {
 			label = "User";
 			gpios = <&gpiob 9 GPIO_ACTIVE_LOW>;

boards/st/stm32vl_disco/stm32vl_disco.dts

@@ -22,10 +22,12 @@
 
 	leds {
 		compatible = "gpio-leds";
+
 		green_led: ld3 {
 			gpios = <&gpioc 9 GPIO_ACTIVE_HIGH>;
 			label = "User LD3";
 		};
+
 		blue_led: ld4 {
 			gpios = <&gpioc 8 GPIO_ACTIVE_HIGH>;
 			label = "User LD4";
@@ -34,6 +36,7 @@
 
 	gpio_keys {
 		compatible = "gpio-keys";
+
 		user_button: button {
 			label = "User";
 			gpios = <&gpioa 0 GPIO_ACTIVE_LOW>;