pinctrl: renesas: checker: Check drive pin conflicts

Check that there is only a single entry for each pin with drive strength
capabilities.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/382206e737710afd3059abe75bc41e324823e657.1640270559.git.geert+renesas@glider.be