nand: Add support for unlock.invert

NAND unlock command allows an invert bit to be set to unlock all but
the selected page range.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
[scottwood@freescale.com: updated docs and added comment about invert bit]
Signed-off-by: Scott Wood <scottwood@freescale.com>