board/atmel: at91sam9x5ek_mmc: add 1M offset for FAT partition

at91sam9x5ek_mmc board was missing in the previous patch adding 1M
offset for FAT partition to solve some boot issues with the ROM code.

Signed-off-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

board/atmel/at91sam9x5ek_mmc/genimage.cfg

@@ -24,6 +24,7 @@ image sdcard.img {
 		partition-type = 0xC
 		bootable = "true"
 		image = "boot.vfat"
+		offset = 1M
 	}
 
 	partition rootfs {