NAND: Add -y option to nand scrub command

This allows the scrub command to scrub without asking the user if he really
wants to scrub the area. Useful in scripts.

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>
Acked-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Scott Wood <scottwood@freescale.com>