spi: spi-mtk-nor: add new clock name 'axi' for spi nor

Some mtk spi nor has dedicated dma(s) inside. Add a new clock name, axi,
for spi nor dma bus clock.
Signed-off-by: Tinghan Shen <tinghan.shen@mediatek.com>
Link: https://lore.kernel.org/r/20211220121825.6446-3-tinghan.shen@mediatek.comSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/mediatek,spi-mtk-nor.yaml

@@ -43,14 +43,19 @@ properties:
     maxItems: 1
 
   clocks:
+    minItems: 2
     items:
       - description: clock used for spi bus
       - description: clock used for controller
+      - description: clock used for nor dma bus. this depends on hardware
+                     design, so this is optional.
 
   clock-names:
+    minItems: 2
     items:
       - const: spi
       - const: sf
+      - const: axi
 
 required:
   - compatible
@@ -84,4 +89,3 @@ examples:
         };
       };
     };
-