boards: st: fix STM3N6 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/stm32n6570_dk/stm32n6570_dk_common.dtsi

@@ -374,6 +374,7 @@ zephyr_udc0: &usbotg_hs1 {
 	width = <800>;
 	height = <480>;
 	pixel-format = <PANEL_PIXEL_FORMAT_RGB_565>;
+
 	display-timings {
 		compatible = "zephyr,panel-timing";
 		de-active = <0>;
@@ -387,6 +388,7 @@ zephyr_udc0: &usbotg_hs1 {
 		hfront-porch = <8>;
 		vfront-porch = <8>;
 	};
+
 	def-back-color-red = <0xFF>;
 	def-back-color-green = <0xFF>;
 	def-back-color-blue = <0xFF>;