flash: nor: mdr: do not mass erase when clearing INFO memory

The MDR parts have two kinds of memory: main (regular memory-mapped
memory for code and data) and "info" (not memory-mapped). When OpenOCD
is requested to erase the info memory block, it should do just that,
instead of erasing everything including main memory.

Change-Id: I498142ca50d4a7b669b7776180b0dbcea63a5328
Reported-by: Eldar Khayrullin <eldar.khayrullin@mail.ru>
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/3853


Tested-by: jenkins
Reviewed-by: Eldar Khayrullin <eldar.khayrullin@mail.ru>
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>