pinctrl/pinconf: partial revert of pin config debugfs

I accidentally left this patch:

commit 6f9e41f4
"pinctrl/pinconfig: add debug interface"

in the tree though the patch was not yet finished.
Revert it partially (we need the infrastructural changes).

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