dt-bindings: memory: mediatek: Add mt8195 smi sub common

Add the binding for smi-sub-common. The SMI block diagram like this:

        IOMMU
         |  |
      smi-common
  ------------------
  |      ....      |
 larb0           larb7   <-max is 8

The smi-common connects with smi-larb and IOMMU. The maximum larbs number
that connects with a smi-common is 8. If the engines number is over 8,
sometimes we use a smi-sub-common which is nearly same with smi-common.
It supports up to 8 input and 1 output(smi-common has 2 output)

Something like:

        IOMMU
         |  |
      smi-common
  ---------------------
  |      |          ...
larb0  sub-common   ...   <-max is 8
      -----------
       |    |    ...   <-max is 8 too.
     larb2 larb5

We don't need extra SW setting for smi-sub-common, only the sub-common has
special clocks need to enable when the engines access dram.

If it is sub-common, it should have a "mediatek,smi" phandle to point to
its smi-common. meanwhile the sub-common only has one gals clock.
Signed-off-by: Yong Wu <yong.wu@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210914113703.31466-3-yong.wu@mediatek.comSigned-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>

Documentation/devicetree/bindings/memory-controllers/mediatek,smi-common.yaml

@@ -38,6 +38,7 @@ properties:
           - mediatek,mt8192-smi-common
           - mediatek,mt8195-smi-common-vdo
           - mediatek,mt8195-smi-common-vpp
+          - mediatek,mt8195-smi-sub-common
 
       - description: for mt7623
         items:
@@ -67,6 +68,10 @@ properties:
     minItems: 2
     maxItems: 4
 
+  mediatek,smi:
+    $ref: /schemas/types.yaml#/definitions/phandle
+    description: a phandle to the smi-common node above. Only for sub-common.
+
 required:
   - compatible
   - reg
@@ -93,6 +98,29 @@ allOf:
             - const: smi
             - const: async
 
+  - if:  # only for sub common
+      properties:
+        compatible:
+          contains:
+            enum:
+              - mediatek,mt8195-smi-sub-common
+    then:
+      required:
+        - mediatek,smi
+      properties:
+        clock:
+          items:
+            minItems: 3
+            maxItems: 3
+        clock-names:
+          items:
+            - const: apb
+            - const: smi
+            - const: gals0
+    else:
+      properties:
+        mediatek,smi: false
+
   - if:  # for gen2 HW that have gals
       properties:
         compatible: