sunxi: Add default partition scheme

sunxi: Add default partition scheme

The partitions variable is especially useful to create a partition table
from U-Boot, either directly from the U-Boot shell, or through flashing
tools like fastboot and its oem format command.

This is especially useful on devices with an eMMC you can't take out to
flash from another system, and booting a Linux system first to flash our
system then is not really practical.

Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>