flash/nor: make all working area pointers local

flash/nor: make all working area pointers local

Working area pointers shouldn't be re-used, so there's no point in storing
them in the flash bank struct. Make all such pointers local.

Change-Id: Iab65b4e8b475fed7fc72fb8928f54590fa69d260
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/865
Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>