pinctrl: sh-pfc: checker: Add check for config register conflicts

Add a helper to verify that register addresses are unique, and use it to
validate config register descriptors.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Link: https://lore.kernel.org/r/20200110131927.1029-5-geert+renesas@glider.be