sunxi_nand_spl: Be smarter about where to look for backup u-boot.bin

sunxi_nand_spl: Be smarter about where to look for backup u-boot.bin

We know when u-boot is written to its own partition, in this case the
layout always is:

eb 0 spl
eb 1 spl-backup
eb 2 u-boot
eb 3 u-boot-backup

eb: erase-block

So if we cannot load u-boot from its primary offset we know exactly where
to look for it.

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