MAINTAINERS: Add PHYTEC Platforms

PHYTEC currently supports five boards in Zephyr, with another in
development. Given the growing interest from our customers and our
commitment to expanding support for PHYTEC platforms, I am
volunteering to take over the maintenance of these boards.

This will help streamline contributions and reduce the workload on
the silicon vendor maintainers.

Signed-off-by: Daniel Schultz <d.schultz@phytec.de>