docs/pinctrl: document debugfs files

Document debugfs directories and files created for pinctrl subsystem.

Suggested-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Drew Fustini <drew@beagleboard.org>
Link: https://lore.kernel.org/r/20210302053059.1049035-5-drew@beagleboard.org


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>