arm64: dts: qcom: sm8250: Rename UART2 node to UART12

The UART12 node has been mistakenly mentioned as UART2. Let's fix that
for both SM8250 SoC and MTP board and also add pinctrl definition for
it.

Fixes: 60378f1a ("arm64: dts: qcom: sm8250: Add sm8250 dts file")
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>