arm: mvebu: ds414: add u-boot,dm-pre-reloc to spi0

Without this U-Boot-specific property, booting on the Synology DS414
(or DS214+) fails in SPL. The spi0 DT node is not scanned, as a result
the SPI flash cannot be found.

Signed-off-by: Ezra Buehler <ezra@easyb.ch>
Reviewed-by: Stefan Roese <sr@denx.de>