NAND: Add nand read.raw and write.raw commands

These commands should work around various "hardware" ECC and BCH methods.

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>
Cc: Scott Wood <scottwood@freescale.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Detlev Zundel <dzu@denx.de>
[scottwood@freescale.com: s/write the page/access the page/]
Signed-off-by: Scott Wood <scottwood@freescale.com>