dt-bindings: mtd: add new compatible strings and improve description

Add "mediatak,mt2712-nor" and "mediatek,mt7622-nor"
for nor flash node's compatible strings.
Explicate the fallback compatible.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Guochun Mao <guochun.mao@mediatek.com>
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@wedev4u.fr>