sunxi_nand_spl: Add support for backup boot partitions

sunxi_nand_spl: Add support for backup boot partitions

The BROM does not care / use bad page markings, instead it deals with
any bad pages in the first erase-block by simply trying to load "boot0"
from the next erase-block.

This commit implements the same strategy for the sunxi spl nand code,
allowing it to boot from the backup boot partition when the main boot
partition is bad (tested by erasing the main boot partition).

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>