arm64: dts: qcom: sdm845: Add UART nodes

This adds nodes for all possible UARTs to sdm845.dtsi. By default
only configure the RX/TX lines with pinctrl. Boards that use UARTs
with flow control can overwrite the configuration in the
<board>.dtsi.

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>