MAINTAINERS: Add Sensry Platforms

Sensry currently supports 2 boards, based on its own ganymed soc
sy1xx (arch RISCV32), in Zephyr.
Given the growing interest from our customers and our commitment
to expanding support for Sensry platforms, I am volunteering to
take over the maintenance of these boards and ganymed family socs.

Signed-off-by: Sven Ginka <s.ginka@sensry.de>