pinctrl: renesas: rzg2l: Improve code for readability

As the RZ/G2L pinctrl driver is extensively utilized by numerous SoCs and
has experienced substantial growth, enhance code readability by
incorporating FIELD_PREP_CONST/FIELD_GET macros wherever necessary.

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/r/20240129135556.63466-2-prabhakar.mahadev-lad.rj@bp.renesas.com


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