MAINTAINERS: add me as a co-maintainer of KIRKWOOD

Signed-off-by: Luka Perkov <luka.perkov@sartura.hr>
CC: Prafulla Wadaskar <prafulla@marvell.com>
CC: Wolfgang Denk <wd@denx.de>
Acked-by: Stefan Roese <sr@denx.de>