serial: msm: add debug UART

serial: msm: add debug UART

Introduce support for early debugging. This relies on the previous stage
bootloader to initialise the UART clocks, when running with U-Boot as
the primary bootloader this feature doesn't work. It will require a way
to configure the clocks before the driver model is available.

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