MAINTAINERS: add entry for Kernel Selftest Framework

Add entry for Kernel Selftest Framework.  Individual tests continue to be
maintained by the maintainers for those areas.

Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>