pinctrl: renesas: rzg2l: Move RZG2L_SINGLE_PIN definition to top of the file

Define `RZG2L_SINGLE_PIN` at the top of the file to clarify its use for
dedicated pins for improved readability.

While at it update the comment for `RZG2L_SINGLE_PIN_PACK` macro and place
it just above the macro for clarity.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
Link: https://lore.kernel.org/r/20240618174831.415583-4-prabhakar.mahadev-lad.rj@bp.renesas.com


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