MAINTAINERS: Promote attie-argentum as Atmel collaborator

@attie-argentum has been collaborating with SAM0 SoC for a few
time. He seems to have good knowledge and it is motivated to
assume more responsabilities on the project. This add
@attie-argentum as a collaborator for Microchip SAM platforms.

Signed-off-by: Gerson Fernando Budke <nandojve@gmail.com>