pinctrl: sh-pfc: checker: Add check for enum ID conflicts

Add a helper to verify that enum IDs are unique, and use it to validate
the enum ID arrays in config register descriptors.

This exposes bugs like those fixed in:
  - commit 805f6357 ("pinctrl: sh-pfc: r8a7778: Fix duplicate
    SDSELF_B and SD1_CLK_B"),
  - commit 884caada ("pinctrl: sh-pfc: sh7734: Fix duplicate
    TCLK1_B"),
  - commit 2a069a92 ("pinctrl: sh-pfc: sh7264: Fix Port K I/O
    Register 0 definition").

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Link: https://lore.kernel.org/r/20200110131927.1029-6-geert+renesas@glider.be