dt-bindings: net: ethernet-phy: add property mac-termination-ohms

Add property mac-termination-ohms in the device tree bindings for selecting
the resistance value of the builtin series termination resistors of the
PHY. Changing the resistance to an appropriate value can reduce signal
reflections and therefore improve signal quality.

Signed-off-by: Dimitri Fedrau <[e-mail]>
Reviewed-by: Rob Herring (Arm) <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

Documentation/devicetree/bindings/net/ethernet-phy.yaml

···
       peak-to-peak specified in ANSI X3.263. When omitted, the PHYs default
       will be left as is.
 
+  mac-termination-ohms:
+    maximum: 200
+    description:
+      The xMII signals need series termination on the driver side to match both
+      the output driver impedance and the line characteristic impedance, to
+      prevent reflections and EMI problems. Select a resistance value which is
+      supported by the builtin resistors of the PHY, otherwise the resistors may
+      have to be placed on board. When omitted, the PHYs default will be left as
+      is.
+
   leds:
     type: object