dt-bindings: pinctrl: renesas,rzn1-pinctrl: documentation

The Renesas RZ/N1 device family PINCTRL node description.

Based on a patch originally written by Michel Pollet at Renesas.

Signed-off-by: Phil Edworthy <phil.edworthy@renesas.com>
Reviewed-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>