Update Nook Color machine 3284 to common Encore name

Machine database already updated:
http://www.arm.linux.org.uk/developer/machines/list.php?id=3284



Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Acked-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Tony Lindgren <tony@atomide.com>