MTD: Add lock/unlock operations for Atmel AT49BV6416

The AT49BV6416 is locked by default, so we really need to provide
at least the unlock() operation for write and erase to work. This
patch implements both ->lock() and ->unlock() and provides a fixup
to install them when an AT49BV6416 chip is detected.

These functions are probably valid on more Atmel chips, but I believe
it's mostly obsolete ones. The AT49BV6416 is in fact obsolete, but
it's used on all current AT32STK1000 development boards.

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>
Signed-off-by: Josh Boyer <jwboyer@gmail.com>