cmd_nand: fix help of nand erase subcommand

Since commit 30486322 (nand erase: .spread, .part, .chip subcommands)
the arguments off and size are no longer optional.

Signed-off-by: Daniel Hobi <daniel.hobi@schmid-telecom.ch>
Cc: Scott Wood <scottwood@freescale.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>