pinctrl: Add pinconf support for BM1880 SoC

Add pinconf support for Bitmain BM1880 SoC. Pinconf support includes
pin bias, slew rate and schmitt trigger. Drive strength support will
be added later.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>