mtd: cfi_cmdset_0002: add CFI detection for SST 38VF640x chips

Add support for SST38VF640x chips in CFI mode.

Signed-off-by: Guillaume LECERF <glecerf@gmail.com>
Signed-off-by: yidong zhang <zhangyd6@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>