nand: Add verification functions

Add nand_verify() and nand_verify_page_oob().  nand_verify() verifies
NAND contents against an arbitrarily sized buffer using ECC while
nand_verify_page_oob() verifies a NAND page's contents and OOB.

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>
Tested-by: Heiko Schocher <hs@denx.de>
Acked-by: Heiko Schocher <hs@denx.de>