flash: nor: mdr: implement read/verify for Info memory

flash: nor: mdr: implement read/verify for Info memory

This adds necessary code to obtain data from the Info region which is
not memory-mapped so can't be read the usual way.

With this "flash read_bank" and "flash verify_bank" commands should
start to work as expected for the Info memory block.

Change-Id: I57e4b80fff577500cfa85954b625fe9c9ff92aa5
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/3227
Tested-by: jenkins
Reviewed-by: Eldar Khayrullin <eldar.khayrullin@mail.ru>
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>