dt-bindings: pinctrl: Add BM1880 pinctrl binding

Add pinctrl binding for Bitmain BM1880 SoC. The SoC is not capable
of handling pinconf, thereby supporting only pinmux and this limitation
is documented.

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