mvebu: turris_omnia: use u-boot-spl-dtb.bin

u-boot-spl.bin and u-boot-spl-dtb.bin are identical when building the
turris_omnia_defconfig. This commit makes Turris Omnia consistent with
all other mvebu boards.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Stefan Roese <sr@denx.de>