ARM: dts: stm32: update led nodes names for stm32f746-eval

Update led nodes names to be aligned with yaml description
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>

arch/arm/boot/dts/stm32mp15xx-dkx.dtsi

@@ -62,7 +62,7 @@ gpu_reserved: gpu@d4000000 {
 
 	led {
 		compatible = "gpio-leds";
-		blue {
+		led-blue {
 			label = "heartbeat";
 			gpios = <&gpiod 11 GPIO_ACTIVE_HIGH>;
 			linux,default-trigger = "heartbeat";