sunxi: disable direct MMC environment

Since the dawn of time for the Allwinner support in mainline U-Boot
we store the environment to the SD card and write directly at
544KB from the beginning of the device. This leads to problems when
the U-Boot proper image grows beyond 504KB and eventually overlaps.
With one release of having the environment preferably in a FAT
partition, let's now turn off the MMC variant fallback, so we get back
all the space we need to implement features.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>