ARM: dts: imx6ul-kontron-sl-common: Fix QSPI NAND node name

Rename QSPI NAND node to 'flash@0' in order to fix the following
dt-schema warning:

spi-flash@0 (spi-nand): $nodename:0: 'spi-flash@0' does not match '^(flash|.*sram|nand)(@.*)?$'

Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

Author: fschrempf

arch/arm/boot/dts/nxp/imx/imx6ul-kontron-sl-common.dtsi

@@ -82,7 +82,7 @@
 	pinctrl-0 = <&pinctrl_qspi>;
 	status = "okay";
 
-	spi-flash@0 {
+	flash@0 {
 		#address-cells = <1>;
 		#size-cells = <1>;
 		compatible = "spi-nand";