mtd: cfi: AMD/Fujitsu compatibles: add panic write support

This allows the mtdoops driver to work on flash chips using the
AMD/Fujitsu compatible command set.

As the code comments note, the locks used throughout the normal code
paths in the driver are ignored, so that the chance of writing out the
kernel's last messages are maximized.

Signed-off-by: Ira W. Snyder <iws@ovro.caltech.edu>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>