MAINTAINERS: Remove Kevin as maintainer of BMIPS generic platforms

The last time Kevin did a review was sometime around 2014,
since then, he has not been active for the BMIPS generic platform
changes.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
[paulburton@kernel.org:
  Drop the non-technical commit message content; Kevin's absence from
  the role is ample reasoning for this change.]
Signed-off-by: Paul Burton <paulburton@kernel.org>