dt-bindings: pincfg-node: Add "output-impedance-ohms" property

On RZ/G2L SoC for Group-B pins, output impedance can be configured. This
patch documents "output-impedance-ohms" property in pincfg-node.yaml so
that other platforms requiring such feature can make use of this property.

Signed-off-by: Lad Prabhakar <[e-mail]>
Reviewed-by: Biju Das <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Reviewed-by: Linus Walleij <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Geert Uytterhoeven <[e-mail]>

Documentation/devicetree/bindings/pinctrl/pincfg-node.yaml

···
     description: enable output on a pin without actively driving it
       (such as enabling an output buffer)
 
+  output-impedance-ohms:
+    description: set the output impedance of a pin to at most X ohms
+
   output-low:
     type: boolean
     description: set the pin to output mode with low level