NAND: Make page, erase, oob size available via cmd_nand

The "nand info" and "nand device" now set shell/environment variables:
	nand_writesize ... nand page size
	nand_oobsize ..... nand oob area size
	nand_erasesize ... nand erase block size

Also, the "nand info" command now displays this info.

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: removed unnecessary memsets]
Signed-off-by: Scott Wood <scottwood@freescale.com>