pinctrl: renesas: rzg2l: Validate pins before setting mux function

Ensure only valid pins are configured by validating pin mappings before
setting the mux function.

Rename rzg2l_validate_gpio_pin() to rzg2l_validate_pin() to reflect its
broader purpose validating both GPIO pins and muxed pins. This helps
avoid invalid configurations.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/20250616132750.216368-1-prabhakar.mahadev-lad.rj@bp.renesas.com


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