pinctrl: pinconf: fix compilation error if PINCONF is not selected

When we compile pinctrl layer for platforms without CONFIG_PINCONF, we get
following compilation errors:

drivers/built-in.o: In function `pinctrl_show':
linux-2.6/drivers/pinctrl/core.c:1116: undefined
reference to `pinconf_show_setting'
drivers/built-in.o: In function `pinctrl_maps_show':
linux-2.6/drivers/pinctrl/core.c:1071: undefined
reference to `pinconf_show_map'
drivers/built-in.o: In function `pinctrl_init_device_debugfs':
linux-2.6/drivers/pinctrl/core.c:1224: undefined
reference to `pinconf_init_device_debugfs'
make[1]: *** [.tmp_vmlinux1] Error 1

This patch fixes this.

Signed-off-by: Viresh Kumar <viresh.kumar@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>