logging: backend_uart: handle runtime PM

Allow the log_backend_uart char_out implementation to integrate with
'Device Runtime Power Management'.

Without using runtime PM, it is impossible for the application to manage
the power state of a UART instance, given that any module in the
application could theoretically LOG_* or printk at any point in time.

This removes the need to manually call `pm_device_state_set` when the
lowest power states must be reached, while still allowing log messages
to be output.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>