mtd: spi-nor: Add support for gd25q16

Add GigaDevice GD25Q16 (16M-bit) to supported list.

Signed-off-by: Kamal Dasu <kdasu.kdev@gmail.com>
Acked-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>