dt-bindings: net: phy: Document LED inactive high impedance mode

Document LED inactive high impedance mode to set the LED to require high
impedance configuration to be turned OFF.

Signed-off-by: Christian Marangi <[e-mail]>
Reviewed-by: Andrew Lunn <[e-mail]>
Acked-by: Lee Jones <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

Documentation/devicetree/bindings/leds/common.yaml

···
       Makes LED active low. To turn the LED ON, line needs to be
       set to low voltage instead of high.
 
+  inactive-high-impedance:
+    type: boolean
+    description:
+      Set LED to high-impedance mode to turn the LED OFF. LED might also
+      describe this mode as tristate.
+
   # Required properties for flash LED child nodes:
   flash-max-microamp:
     description: