arm64: dts: qcom: sdm845-oneplus-common: enable debug UART

A labelled diagram showing the location of the Rx and Tx testpoints for
the OnePlus 6 is available on the postmarketOS wiki:

https://wiki.postmarketos.org/wiki/Serial_debugging:Cable_schematics



The device uses 1.8v UART at a baud rate of 115200, bootloader output is
also available here.

Signed-off-by: Caleb Connolly <caleb@connolly.tech>
Link: https://lore.kernel.org/r/20210720153125.43389-3-caleb@connolly.tech


Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>