pinctrl: single: support generic pinconf

Support the operation of generic pinconf. The supported config arguments
are INPUT_SCHMITT, INPUT_SCHMITT_ENABLE, DRIVE_STRENGHT, BIAS_DISABLE,
BIAS_PULLUP, BIAS_PULLDOWN, SLEW_RATE.

Signed-off-by: Haojian Zhuang <haojian.zhuang@linaro.org>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>