nitrogen6x: change maintainer

Troy Kisky will be maintaining the Nitrogen6x board going forward.
Signed-off-by: NEric Nelson <eric.nelson@boundarydevices.com>
Acked-by: NTroy Kisky <troy.kisky@boundarydevices.com>

board/boundary/nitrogen6x/MAINTAINERS

 NITROGEN6X BOARD
-M:	Eric Nelson <eric.nelson@boundarydevices.com>
+M:	Troy Kisky <troy.kisky@boundarydevices.com>
 S:	Maintained
 F:	board/boundary/nitrogen6x/
 F:	include/configs/nitrogen6x.h