board: siemens: Add support for SIMATIC IOT2050 devices

This adds support for the IOT2050 Basic and Advanced devices. The Basic
used the dual-core AM6528 GP processor, the Advanced one the AM6548 HS
quad-core version.

Both variants are booted via a Siemens-provided FSBL that runs on the R5
cores. Consequently, U-Boot support is targeting the A53 cores. U-Boot
SPL, ATF and TEE have to reside in SPI flash.

Full integration into a bootable image can be found on
https://github.com/siemens/meta-iot2050



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

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