dt-bindings: mmc: mtk-sd: add tuning steps related property

Add 'mediatek,tuning-steps' setting. This property will give MSDC
a chance to extend tuning steps up to 64. With more tuning steps,
MSDC may achieve a more optimal calibration result, thus avoiding
potential CRC issues.

Reviewed-by: Krzysztof Kozlowski <[e-mail]>
Signed-off-by: Axe Yang <[e-mail]>
Reviewed-by: AngeloGioacchino Del Regno <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Ulf Hansson <[e-mail]>

Documentation/devicetree/bindings/mmc/mtk-sd.yaml

···
     minimum: 0
     maximum: 7
 
+  mediatek,tuning-step:
+    $ref: /schemas/types.yaml#/definitions/uint32
+    description:
+      Some SoCs need extend tuning step for better delay value to avoid CRC issue.
+      If not present, default tuning step is 32. For eMMC and SD, this can yield
+      satisfactory calibration results in most cases.
+    enum: [32, 64]
+    default: 32
+
   resets:
     maxItems: 1