ARM: dts: imx27-apf27dev: Fix LED name

Per leds-gpio.yaml, the led names should start with 'led'.

Change it to fix the following dt-schema warning:

imx27-apf27dev.dtb: leds: 'user' does not match any of the regexes: '(^led-[0-9a-f]$|led)', 'pinctrl-[0-9]+'
	from schema $id: http://devicetree.org/schemas/leds/leds-gpio.yaml#Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm/boot/dts/nxp/imx/imx27-apf27dev.dts

@@ -47,7 +47,7 @@ leds {
 		pinctrl-names = "default";
 		pinctrl-0 = <&pinctrl_gpio_leds>;
 
-		user {
+		led-user {
 			label = "Heartbeat";
 			gpios = <&gpio6 14 GPIO_ACTIVE_HIGH>;
 			linux,default-trigger = "heartbeat";