configs: sama5d2_xplained: set default FAT env location to SD-Card

For sama5d2_xplained_mmc_defconfig, we have the following layout for SD-Card:
partition 1: FAT: contains bootstrap binary (second level bootloader),
U-boot, U-boot env, kernel, dtb
partition 2: EXT4: Rootfs.

Add to defconfig CONFIG_ENV_FAT_DEVICE_AND_PART to have environment
by default on SD-Card, to align with our demo layout.

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>