pinctrl: sh-pfc: checker: Add function GPIO checks

Add checks for legacy function GPIO descriptors:
  1. Function GPIOs must have a name,
  2. Names must be unique,
  3. Enum ID values must be unique.

This exposes bugs like those fixed in
  - commit 884caada ("pinctrl: sh-pfc: sh7734: Fix duplicate
    TCLK1_B"),
  - commit 55b1cb1f ("pinctrl: sh-pfc: sh7264: Fix CAN function
    GPIOs"),
  - commit 02aeb2f2 ("pinctrl: sh-pfc: sh7269: Fix CAN function
    GPIOs"),
  - commit db9c0727 ("sh: sh7264: Remove bogus SSU GPIO function
    definitions"),
  - commit b4fba344 ("sh: sh7269: Remove bogus SSU GPIO function
    definitions").

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Link: https://lore.kernel.org/r/20200110131927.1029-14-geert+renesas@glider.be