sf: winbond: add support W25Q64 parts

Adds support for Winbond's W25Q64 SPI flash. These devices are used on
(among others) Xilinx' SP601 and SP605 Spartan-6 evaluation boards.
Tested with "sf" commands.

Signed-off-by: Graeme Smecher <graeme.smecher@mail.mcgill.ca>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>