ARM: mvebu: fix unit address on armada-390-db flash

The unit address needs to be changed to match the reg property:

arch/arm/boot/dts/marvell/armada-390-db.dts:84.10-106.4: Warning (spi_bus_reg): /soc/spi@10680/flash@1: SPI bus unit address format error, expected "0"

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>