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>