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

Improve the checks for pin group descriptors:
  1. Introduce a local variable for the current group, to make the
     checks easier to read,
  2. Pin group 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/20200110131927.1029-9-geert+renesas@glider.be