samples: net: latmon: latency monitor tool

Supports the EVL/Xenomai4 benchmarking tool as described in the
documentation. The benchmarking tool is accessed via the latmon service.

This code uses the J2 socket on FRDMk64-F:
             PIN_20: tx pulse to SUT
             PIN_18: rx ack from SUT

The client code running on the SUT shall monitor the falling edge of
PIN_20.

Example usage from the latmus client running Xenomai4:
 $ latmus -I gpiochip2,23,falling-edge  \
          -O gpiochip2,21 -g"histogram" \
          -z broadcast

Signed-off-by: Jorge Ramirez-Ortiz <jorge.ramirez@oss.qualcomm.com>