mtd: m25p80: introduce SST_WRITE flag for SST byte programming

Not all SST devices implement the SST byte programming command.
Some devices (like SST25VF064C) implement only standard m25p80 page
write command.

Now SPI flash devices that need sst_write() are explicitly marked
with new SST_WRITE flag and the decision to use sst_write() is based
on this flag instead of manufacturer id.

Signed-off-by: Krzysztof Mazur <krzysiek@podlesie.net>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>