pinctrl/pinconfig: add debug interface

This update adds a debugfs interface to modify a pin configuration
for a given state in the pinctrl map. This allows to modify the
configuration for a non-active state, typically sleep state.
This configuration is not applied right away, but only when the state
will be entered.

This solution is mandated for us by HW validation: in order
to test and verify several pin configurations during sleep without
recompiling the software.

Signed-off-by: Laurent Meunier <laurent.meunier@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>