arm64: dts: bitmain: Add UART pinctrl support for Sophon Edge

Add pinctrl support for UARTs exposed on the Sophon Edge board.

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