pinctrl: sh-pfc: checker: Add drive strength register checks

Add checks for drive strength register descriptors:
  1. Register addresses must be unique,
  2. Register fields must be non-overlapping,
  3. Referred pins must exist.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Link: https://lore.kernel.org/r/20200110131927.1029-10-geert+renesas@glider.be