dt-bindings: dp83867: add binding for io_impedance_ctrl nvmem cell

We have a board where measurements indicate that the current three
options - leaving IO_IMPEDANCE_CTRL at the reset value (which is
factory calibrated to a value corresponding to approximately 50 ohms)
or using one of the two boolean properties to set it to the min/max
value - are too coarse.

There is no fixed mapping from register values to values in the range
35-70 ohms; it varies from chip to chip, and even that target range is
approximate. So add a DT binding for an nvmem cell which can be
populated during production with a value suitable for each specific
board.
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/devicetree/bindings/net/ti,dp83867.yaml

@@ -31,6 +31,16 @@ properties:
   reg:
     maxItems: 1
 
+  nvmem-cells:
+    maxItems: 1
+    description:
+      Nvmem data cell containing the value to write to the
+      IO_IMPEDANCE_CTRL field of the IO_MUX_CFG register.
+
+  nvmem-cell-names:
+    items:
+      - const: io_impedance_ctrl
+
   ti,min-output-impedance:
     type: boolean
     description: |
@@ -42,9 +52,11 @@ properties:
     description: |
       MAC Interface Impedance control to set the programmable output impedance
       to a maximum value (70 ohms).
-      Note: ti,min-output-impedance and ti,max-output-impedance are mutually
-        exclusive. When both properties are present ti,max-output-impedance
-        takes precedence.
+      Note: Specifying an io_impedance_ctrl nvmem cell or one of the
+        ti,min-output-impedance, ti,max-output-impedance properties
+        are mutually exclusive. If more than one is present, an nvmem
+        cell takes precedence over ti,max-output-impedance, which in
+        turn takes precedence over ti,min-output-impedance.
 
   tx-fifo-depth:
     $ref: /schemas/types.yaml#/definitions/uint32