imx6ul: geam6ul: Add MAINTAINERS for nand_defconfig

Add Jagan as MAINTAINERS of configs/imx6ul_geam_nand_defconfig

Cc: Stefano Babic <sbabic@denx.de>
Cc: Matteo Lisi <matteo.lisi@engicam.com>
Cc: Michael Trimarchi <michael@amarulasolutions.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>