dt-bindings: arm: ti: Add bindings for Siemens IOT2050 boards

These boards are based on AM6528 GP and AM6548 HS SOCs.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/173ce7d928ed9f352af7673dd44c6c76a1466eb5.1615473223.git.jan.kiszka@siemens.com