pinctrl: renesas: r8a7794: Add range checking to .pin_to_pocctrl()

The .pin_to_pocctrl() implementation for R-Car E2 does not perform a
full range check, unlike on all other SoCs.  Add the range check, so the
checker can validate better the consistency of the pin control tables.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/d23767ad7152327654192d7191f4b8ae19493966.1640269510.git.geert+renesas@glider.be