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

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 <[email protected]>
Cc: Scott Wood <[email protected]>
Cc: Stefano Babic <[email protected]>
Cc: Wolfgang Denk <[email protected]>
Cc: Detlev Zundel <[email protected]>
[[email protected]: removed unnecessary memsets]
Signed-off-by: Scott Wood <[email protected]>