sunxi/nand: Add support to the SPL for loading u-boot from internal NAND memory

This commit adds support to the sunxi SPL to load u-boot from the internal
NAND. Note this only adds support to access the boot partitions to load
u-boot, full NAND support to load the kernel, etc. from the nand data
partition will come later.

Signed-off-by: Roy Spliet <r.spliet@ultimaker.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>