[U-Boot] Proposal to add NAND-boot support for Sunxi SPL

The following patches take the work by Daniel Kochmánski, and make some heavy modifications for readability and functionality, based on Boris Brezillon's Linux driver. Tested on an Olimex Lime w/ A20. Patches are sent as RFC. Open questions: - Config options added are partially NAND-chip specific. Some options can be autodetected based on the NAND ID, others require either brute-forcing or config options like these. Do they belong in sunxi-common? Should we make a Kconfig option for this? If bikeshedding is desired, are defines in sunxi-common.h good enough for now? - Style is mostly kernel-like. Satisfied? - Daniel: do you think we can work from here?

Please comment away!

Roy