pinctrl: sh-pfc: Validate fixed-size field widths at build time

Add a build-time check, to ensure the register and field widths in
descriptors for config registers with fixed-width fields are sane.
This helps catching bugs early.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>