boards: st: fix STM32F2 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 <[email protected]>

Author: etienne-lms

boards/st/nucleo_f207zg/nucleo_f207zg.dts

@@ -23,14 +23,17 @@
 
 	leds: leds {
 		compatible = "gpio-leds";
+
 		green_led_1: led_1 {
 			gpios = <&gpiob 0 GPIO_ACTIVE_HIGH>;
 			label = "User LD1";
 		};
+
 		blue_led_1: led_2 {
 			gpios = <&gpiob 7 GPIO_ACTIVE_HIGH>;
 			label = "User LD2";
 		};
+
 		red_led_1: led_3 {
 			gpios = <&gpiob 14 GPIO_ACTIVE_HIGH>;
 			label = "User LD3";
@@ -44,10 +47,12 @@
 			pwms = <&pwm3 3 PWM_MSEC(20) PWM_POLARITY_NORMAL>;
 			label = "Green PWM LED";
 		};
+
 		blue_pwm_led: led_pwm_2 {
 			pwms = <&pwm4 2 PWM_MSEC(20) PWM_POLARITY_NORMAL>;
 			label = "Blue PWM LED";
 		};
+
 		red_pwm_led: led_pwm_0 {
 			pwms = <&pwm12 1 PWM_MSEC(20) PWM_POLARITY_NORMAL>;
 			label = "Red PWM LED";
@@ -56,6 +61,7 @@
 
 	gpio_keys {
 		compatible = "gpio-keys";
+
 		user_button: button {
 			label = "User";
 			gpios = <&gpioc 13 GPIO_ACTIVE_HIGH>;