mtd: m25p80: Add support for two new Spansion SPI devices (S25FL-K)

This patch adds support for Spansion S25FL016K and S25FL064K SPI flash.
It has been tested with physical devices. Note that both parts exhibit
a Winbond manufacturer ID so they might also be added to that section.

Signed-off-by: Gernot Hoyler <Gernot.Hoyler@spansion.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>