arm64: dts: ls208xa-rdb: Pass unit name to SPI flash node

Pass unit name to SPI flash node to match its 'reg' value and
also avoid the following DTC warnings:

arch/arm64/boot/dts/freescale/fsl-ls2080a-rdb.dtb: Warning (unit_address_vs_reg): /soc/dspi@2100000/n25q512a: node has a reg or ranges property, but no unit name
arch/arm64/boot/dts/freescale/fsl-ls2088a-rdb.dtb: Warning (unit_address_vs_reg): /soc/dspi@2100000/n25q512a: node has a reg or ranges property, but no unit name

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>