pinctrl: renesas: rzg2l: Adapt function number for RZ/G3S

pinctrl: renesas: rzg2l: Adapt function number for RZ/G3S

On RZ/G3S PFC register allow setting 8 functions for individual ports
(function1 to function8). For function1 the register need to be
configured with 0, for function8 the register need to be configured with
7.  We cannot use zero based addressing when requesting functions from
different code places as the documentation
(RZG3S_pinfunction_List_r1.0.xlsx) states explicitly that function0 is
GPIO.

Add a new member to struct rzg2l_hwcfg that will keep the offset that
needs to be substracted before applying a value to a PFC register.

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/20230929053915.1530607-17-claudiu.beznea@bp.renesas.com
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>