pinctrl: sh-pfc: checker: Improve pin function checks

Improve the checks for pin function descriptors:
  1. Merge declaration and assignment of the local variable for the
     current pin function,
  2. Pin function names must be unique.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Link: https://lore.kernel.org/r/20200110131927.1029-8-geert+renesas@glider.be