ARM: dts: mvebu: fix SPI device node

dtc warns about a mismatched address:

arch/arm/boot/dts/armada-385-atl-x530.dts:171.14-199.4: Warning (spi_bus_reg): /soc/spi@10680/spi-flash@0: SPI bus unit address format error, expected "1"

I assume the "reg" property is correct here, so adjust the unit address
accordingly.

Fixes: c6dfc019 ("ARM: dts: mvebu: Add device tree for ATL-x530 Board")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reported-by: kernelci.org bot <bot@kernelci.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>