arm64: dts: qcom: sdm845: commonize bluetooth UART pinmux

The 4-pin configuration for UART6 is used for all or almost all SDM845
devices with built in Bluetooth. Move the pinmux configuration to
sdm845.dtsi in preparation to be removed from individual devices in
future patches.

Suggested-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Caleb Connolly <caleb@connolly.tech>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20221016172944.1892206-2-kc@postmarketos.org