pinctrl: renesas: rzg2l: Add pin configuration support for pinmux groups

On RZ/G3S different Ethernet pins need to be configured with different
settings (e.g. power-source needs to be set, RGMII TXC and TX_CTL pins
need output-enable).  Adjust the driver to allow specifying pin
configuration for pinmux groups.  With this, DT settings like the
following are taken into account by the driver:

    eth0_pins: eth0 {
	    tx_ctl {
		    pinmux = <RZG2L_PORT_PINMUX(1, 1, 1)>;  /* ET0_TX_CTL */
		    power-source = <1800>;
		    output-enable;
		    drive-strength-microamp = <5200>;
	    };
    };

Signed-off-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20231207070700.4156557-5-claudiu.beznea.uj@bp.renesas.com
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>