arm64: dts: bitmain: Add pinctrl support for BM1880 SoC

Add pinctrl support for Bitmain BM1880 SoC. This SoC only supports
pinmuxing and the pinctrl registers are part of the sctrl block.

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