ARM: opos6ul: make the board boot again

ARM: opos6ul: make the board boot again

Commit 9faa43c4b5e5 ("ARM: dts: i.MX6UL: U-Boot specific dts for u-boot,
dm-spl") removes the u-boot,dm-spl properties from the imx6ul.dtsi file
and breaks the OPOS6UL board.
Add the u-boot,dm-spl properties into *-u-boot.dts files to make the
board boot again.

Fixes: commit 9faa43c4b5e5 ("ARM: dts: i.MX6UL: U-Boot specific dts for u-boot, dm-spl")
Signed-off-by: Sébastien Szymanski <sebastien.szymanski@armadeus.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Stefano Babic <sbabic@denx.de>