mtd: spi-nor: fix flags for s25fl128s

The Spansion S25FL128S also supports dual read mode.
In addition remove flag SECT_4K. 4K erases are supported,
but not uniformly.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Jagan Teki <jteki@openedev.com>
Acked-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>