pinctrl: renesas: Annotate sentinels in tables

It does not make sense to have a comma after a sentinel, as any new
elements must be added before the sentinel.  Hence drop all such commas.

Add comments to clarify the purpose of the empty elements.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/845f1d8285dd44522af1b0f429d4c6bd4759eb9e.1678272180.git.geert+renesas@glider.be