ARM: dts: mvebu: fix reference to a390 spi controller

The SPI controller in the arch/arm/boot/dts/armada-39x.dtsi file has moved
to a different location in the hierarchy, which breaks the overrides
in the board specific file:

Warning (reg_format): "reg" property in /soc/internal-regs/spi@10680/spi-flash@1 has invalid length (4 bytes) (#address-cells == 2, #size-cells == 1)
Warning (avoid_default_addr_size): Relying on default #address-cells value for /soc/internal-regs/spi@10680/spi-flash@1

This changes the board to reference the spi controller by its label
(which has not changed) rather than the full path.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 0160a4b6

 ("ARM: dts: mvebu: A37x/XP/38x/39x: Move SPI controller nodes into 'soc' node")
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>