board/qualcomm: add debug config fragments for some SoCs

We already have some documentation describing how to enable debug UART
for Qualcomm SoCs. However the UART address varies per-soc... Add some
config fragments to enable debug UART for few well supported SoCs.

These can be used like:

$ make qcom_defconfig debug-sdm845.config

Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>