MAINTAINERS: Add maintainer for MIPS Baikal-T1 platform code

Add myself as a maintainer of the MIPS Baikal-T1 platform-specific
drivers. The arch-code hasn't been submitted yet, but will be soon enough.
Until then it's better to have the already available drivers marked as
maintained.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>