MAINTAINERS: Add network applications and update tinycrypt sections

Add the Network Applications sections covering the following:
- nats clients
- paho_mqtt_clients
- dns_client

Update the tinycrypt section:
- Add the tests/crypto directory. This directory includes one mbedtls
  test and the remaining are tinycrypt tests.

For more information, see:

    https://gerrit.zephyrproject.org/r/3411
    https://gerrit.zephyrproject.org/r/3412



Change-Id: I389cde59cca0f429baffc1a44b43a6a87b5093fd
Signed-off-by: Flavio Santes <flavio.santes@intel.com>