arm: mvebu: clearfog: add u-boot, dm-spl tag for spi

This is required so SPL and u-boot can boot from spi
flash devices that use the dm drivers.

Signed-off-by: Jon Nettleton <jon@solid-run.com>
[baruch: use -u-boot.dtsi]
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Stefan Roese <sr@denx.de>