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 four variants, Basic and Advanced, each as
product generation 1 (SR1.0) and 2 (SR2.x), so there are separate dts
files needed. 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>