mtd: cfi_cmdset_0002: add CFI detection for SST 39VF{16, 32}xx chips

SST 39VF{16,32}xx chips use the 0x0701 command set, fully compatible
with the AMD one. This patch adds support for detecting them in CFI
mode.

Signed-off-by: Guillaume LECERF <glecerf@gmail.com>
Reviewed-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>