arm: dts: Add IOT2050 device tree files

Prepares for the addition of the IOT2050 board which is based on the TI
AM65x. The board comes in two variants, Basic and Advanced, so there are
separate dts files. Furthermore, the SPL has its own device tree.

Based on original board support by Le Jin, Gao Nian and Chao Zeng.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>