dt-bindings: pinctrl: renesas: Document RZ/G3S SoC

Add documentation for the pin controller found on the Renesas RZ/G3S
(R9A08G045) SoC.  Compared to RZ/G2{L,UL}, RZ/G3S has 82 general-purpose
IOs, no slew rate and output impedance support, and more values for
drive strength which needs to be expressed in microamp.

Signed-off-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20230929053915.1530607-21-claudiu.beznea@bp.renesas.com


Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>