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>