Use dts support from U-Boot via OF_SEPARATE instead of depending from opensbi.

This would help to make the necessary changes in drivers and device trees
in U-Boot tree itself. This feature would also be helpful to not pass
dtb during opensbi builds.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>
Signed-off-by: Rick Chen <rick@andestech.com>