dt-bindings: pinctrl: renesas,rzg2l-pinctrl: Add description for power-source property

Add description for "power-source" property mentioning the values in
enum are in millivolt.
Suggested-by: Pavel Machek <pavel@denx.de>
Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20211222145901.23661-1-prabhakar.mahadev-lad.rj@bp.renesas.comSigned-off-by: Geert Uytterhoeven <geert+renesas@glider.be>

Documentation/devicetree/bindings/pinctrl/renesas,rzg2l-pinctrl.yaml

@@ -76,6 +76,7 @@ additionalProperties:
         output-impedance-ohms:
           enum: [ 33, 50, 66, 100 ]
         power-source:
+          description: I/O voltage in millivolt.
           enum: [ 1800, 2500, 3300 ]
         slew-rate: true
         gpio-hog: true