samples: Add an SWO logger configuration fragment

The subsys/logging/logger example can be used to demonstrate how to
use SWO for logging. Add a configuration fragment that enables the
logger's SWO backend. The default SWO frequency in this example is set
to 2 MHz to ensure a well-defined output bit rate instead of the
highest possible.

Signed-off-by: Andreas Sandberg <andreas@sandberg.uk>