Documentation: mtk-quadspi: update DT bindings

Add "mediatek,mt2701-nor" for nor flash node's compatible.

Signed-off-by: Guochun Mao <guochun.mao@mediatek.com>
Acked-by: John Crispin <john@phrozen.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>