flash/nor/mdr: add docs, remove memory leak on probe()

This adds the mandatory Info documentation for the driver as well as
the usage field.

As a clean up, this also includes freeing of the allocated memory
which results in a memory leak if probe is invoked multiple times.

Valgrind-tested.

Reported by Dmitry Shpak.

Change-Id: I2b1d9b9e8b069c6665b11d880b40ce19a1b26ce6
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2694


Tested-by: jenkins
Reviewed-by: Дмитрий Шпак <disona@yandex.ru>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>