mtd: mtk-nor: set controller's address width according to nor flash

When nor's size larger than 16MByte, nor's address width maybe
set to 3 or 4, and controller should change address width according
to nor's setting.

Signed-off-by: Guochun Mao <guochun.mao@mediatek.com>
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>