dt-bindings: Add Siemens vendor prefix

Add prefix for Siemens AG.

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/4750c25ded8d1d5791c14b0e7b450a2a918eea36.1615473223.git.jan.kiszka@siemens.com