sunxi: Bring back SD card as MMC device 0

Commit 2243d19e ("mmc: mmc-uclass: Use dev_seq() to read aliases
node's index") now actually enforces U-Boot's device enumeration policy,
where explicitly named devices come first, then any other non-named
devices follow, without filling gaps.

For quite a while we have had an "mmc1 = &mmc2;" alias in our
sunxi-u-boot.dtsi, which now leads to the problem that the SD card
(which was always mmc device 0) now gets to be number 2.
This breaks quite some boot scripts, including our own distro boot
commands, and some other features looking at $mmc_bootdev, also
fastboot.

Just add an explicit mmc0 alias in the very same file to fix this and
restore the old behaviour.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reported-by: Samuel Holland <samuel@sholland.org>
Tested-by: Simon Baatz <gmbnomis@gmail.com>