pinctrl: renesas: checker: Add reserved field checks

Add checks for discovering registers with reserved fields that could
benefit from being described using variable-width reserved field
shorthands, reducing kernel size.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/f5a5159ba7b396e6f09dd3f23c864a74ed8e342d.1649865241.git.geert+renesas@glider.be