mtd: spi-nor: add subsector flag to n25q128a

Micron n25q128axx support subsector (4K) erase so let's update the flags.
Tested on n25q128a13.

Signed-off-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>