arm: dts: Introduce am69-sk u-boot dts files

Introduce the base dts files needed for u-boot or to augment the linux
dtbs for use in the u-boot-spl and u-boot binaries.

Signed-off-by: Dasnavis Sabiya <sabiya.d@ti.com>
Signed-off-by: Apurva Nandan <a-nandan@ti.com>
Reviewed-by: Neha Malcom Francis <n-francis@ti.com>
Tested-by: Marcel Ziswiler <marcel.ziswiler@toradex.com> # AM69-SK