configs: agilex5: Enable FAT-based environment storage in defconfig

configs: agilex5: Enable FAT-based environment storage in defconfig

Enable storing the U-Boot environment in a FAT filesystem for Agilex5.

This allows the board to read and write environment variables from the
first partition of the first device formatted with FAT, in addition to the
existing UBI-based environment configuration.

Signed-off-by: Alif Zakuan Yuslaimi <alif.zakuan.yuslaimi@altera.com>
Reviewed-by: Tien Fong Chee <tien.fong.chee@altera.com>