doc: dt: mtd: add documentation for Mediatek spi-nor controller

Add device tree binding documentation for serial flash with
Mediatek serial flash controller

Signed-off-by: Bayi Cheng <bayi.cheng@mediatek.com>
Acked-by: Rob Herring <robh@kernel.org>
[Brian: fixed up language]
Signed-off-by: Brian Norris <computersforpeace@gmail.com>