imx: fix missing MAINTAINERS pico boards

Add missing files pico-nymph-imx7d_defconfig and
c>onfigs/pico-dwarf-imx7d_defconfig

Signed-off-by: Stefano Babic <sbabic@denx.de>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
CC: Fabio Estevam <festevam@gmail.com>
CC: Otavio Salvador <otavio@ossystems.com.br>