dt-bindings: convert mtk-quadspi binding doc for spi-mtk-nor

spi-mtk-nor is a driver to replace mtk-quadspi and they have almost
the same device-tree bindings. Reuse this binding documentation and
convert it for new driver:

1. "Mediatek SoCs" -> "Mediatek ARM SoCs" because MTK MIPS SoCs
   use different controllers.
2. document "interrupts" as a required property because it's
   available on all SoCs with this controller and new driver takes
   advantages of it. It's implemented as optional only to maintain
   backward compatibility.
3. add a dummy interrupt binding in example.

Signed-off-by: Chuanhong Guo <gch981213@gmail.com>
Link: https://lore.kernel.org/r/20200306085052.28258-4-gch981213@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>

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