mtd: m25p80: Add Spansion S25FL256S

These are 32MiB parts which use a slightly different 4-byte enable
sequence from Macronix.

Default to the Spansion 4-byte scheme in set_4byte(), as it is more
likely to be copied by other vendors.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>