drivers mtd: spi-nor: add Winbond W25Q20 variants

Winbond W25Q20BW devices are used in 4/5th generation Kindle ebook readers.
Add this spi-nor device and the similar W25Q20 devices to the list of known
devices.

Signed-off-by: Alexander Kurz <akurz@blala.de>
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>