ARM: at91: sama5: add support for CONFIG_ENV_IS_IN_MMC

If defined CONFIG_ENV_IS_IN_MMC, then u-boot environment is saved in
mmc's raw sectors. Otherwise, u-boot environment is saved as a file:
uboot.env.

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Reviewed-by: Bo Shen <voice.shen@gmail.com>