Update MAINTAINERS

Add Karthik B S.

Cc: Bhanuprakash Modem <bhanuprakash.modem@gmail.com>
Cc: Petri Latvala <adrinael@adrinael.net>
Cc: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Cc: Ashutosh Dixit <ashutosh.dixit@intel.com>
Cc: Karthik B S <karthik.b.s@intel.com>
Signed-off-by: Kamil Konieczny <kamil.konieczny@linux.intel.com>
Acked-by: Karthik B S <karthik.b.s@intel.com>
Acked-by: Petri Latvala <adrinael@adrinael.net>