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>

board/atmel/at91sam9260ek/MAINTAINERS

@@ -10,6 +10,7 @@ F:	configs/at91sam9g20ek_2mmc_nandflash_defconfig
 F:	configs/at91sam9g20ek_dataflash_cs0_defconfig
 F:	configs/at91sam9g20ek_dataflash_cs1_defconfig
 F:	configs/at91sam9g20ek_mmc_defconfig
+F:	configs/at91sam9g20ek_2mmc_defconfig
 F:	configs/at91sam9g20ek_nandflash_defconfig
 F:	configs/at91sam9xeek_dataflash_cs0_defconfig
 F:	configs/at91sam9xeek_dataflash_cs1_defconfig