pico-imx6ul: MAINTAINERS: Add pico-dwarf entry

pico-dwarf-imx6ul_defconfig does not have an entry in MAINTAINERS file,
so add it to avoid a build warning.

Reported-by: Stefano Babic <sbabic@denx.de>
Signed-off-by: Fabio Estevam <festevam@gmail.com>