ARM: dts: imx50-evk: Use generic node name for SPI NOR flash

Node names should be generic, so use 'flash' as the SPI NOR flash node
name.

This fixes the following schema warning:

imx53-smd.dtb: m25p32@1: $nodename:0: 'm25p32@1' does not match '^(flash|.*sram|nand)(@.*)?$'
	from schema $id: http://devicetree.org/schemas/mtd/jedec,spi-nor.yaml#Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm/boot/dts/nxp/imx/imx50-evk.dts

@@ -23,7 +23,7 @@ &cspi {
 	cs-gpios = <&gpio4 11 GPIO_ACTIVE_LOW>, <&gpio4 13 GPIO_ACTIVE_LOW>;
 	status = "okay";
 
-	flash: m25p32@1 {
+	flash: flash@1 {
 		#address-cells = <1>;
 		#size-cells = <1>;
 		compatible = "m25p32", "jedec,spi-nor";