at91sam9260ek: add missing files to MAINTAINERS

This fixes the following genboardscfg.py warnings:

---8<---
WARNING: no status for 'at91sam9g20ek_2mmc'
WARNING: no maintainers for 'at91sam9g20ek_2mmc'
--->8---

Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>
1 file changed