pinctrl: renesas: checker: Add pin group sharing checks

Add checks for discovering pin groups that could share pin data, as
sharing reduces kernel size.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/e16fdf729156e13b591d2f082f2cc934da580074.1640270559.git.geert+renesas@glider.be