pinctrl: pinctrl-single: add debugfs pin h/w state info

Adds support for displaying the individual pin h/w config state.

Signed-off-by: Matt Porter <mporter@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>