<h3>Contribution description</h3>
<p>This PR make <code>RTT_FREQUENCY</code> configurable for these platforms as well as defining there max an min values.</p>
<p>I didn't move this to <code>periph_cpu</code> because they could use an external oscilotr for the RTT which would make it a <code>BOARD</code> configuration.</p>
<p>Same default have been kept.</p>
<h3>Testing procedure</h3>
<ul>
<li>base test works <code>BOARD=nrf52dk make -C tests/periph_rtt flash test -j3</code></li>
</ul>
<pre><code>main(): This is RIOT! (Version: 2020.07-devel-412-g409185-pr_nrf5x_rtt_conf)

RIOT RTT low-level driver test
This test will display 'Hello' every 5 seconds

Initializing the RTT driver
RTT now: 0
Setting initial alarm to now + 5 s (5120)
Done setting up the RTT, wait for many Hellos
Hello
Hello
Hello
Hello
Hello
</code></pre>
<ul>
<li>can change frequency <code>CFLAGS=-DRTT_FREQUENCY=32768 BOARD=nrf52dk make -C tests/periph_rtt flash test -j3</code></li>
</ul>
<pre><code>main(): This is RIOT! (Version: 2020.07-devel-412-g409185-pr_nrf5x_rtt_conf)

RIOT RTT low-level driver test
This test will display 'Hello' every 5 seconds

Initializing the RTT driver
RTT now: 0
Setting initial alarm to now + 5 s (163840)
Done setting up the RTT, wait for many Hellos
Hello
Hello
Hello
Hello

</code></pre>
<h3>Issues/PRs references</h3>
<p>Split from # <a class="issue-link js-issue-link" data-error-text="Failed to load title" data-id="600181973" data-permission-text="Title is private" data-url="https://github.com/RIOT-OS/RIOT/issues/13874" data-hovercard-type="pull_request" data-hovercard-url="/RIOT-OS/RIOT/pull/13874/hovercard" href="https://github.com/RIOT-OS/RIOT/pull/13874">#13874</a></p>

<hr>

<h4>You can view, comment on, or merge this pull request online at:</h4>
<p>  <a href='https://github.com/RIOT-OS/RIOT/pull/14025'>https://github.com/RIOT-OS/RIOT/pull/14025</a></p>

<h4>Commit Summary</h4>
<ul>
  <li>boards/common/nrf5x: add configurable RTT_FREQUENCY</li>
</ul>

<h4>File Changes</h4>
<ul>
  <li>
    <strong>M</strong>
    <a href="https://github.com/RIOT-OS/RIOT/pull/14025/files#diff-1c6a0965e8f28ca0c3bc9f300cff9766">boards/common/nrf51/include/cfg_rtt_default.h</a>
    (16)
  </li>
  <li>
    <strong>M</strong>
    <a href="https://github.com/RIOT-OS/RIOT/pull/14025/files#diff-a5529d70ea141594c2b827e12305f23a">boards/common/nrf52/include/cfg_rtt_default.h</a>
    (15)
  </li>
  <li>
    <strong>M</strong>
    <a href="https://github.com/RIOT-OS/RIOT/pull/14025/files#diff-80a8652fb7de36d2dc86633b4d7f59a4">cpu/nrf5x_common/periph/rtt.c</a>
    (4)
  </li>
</ul>

<h4>Patch Links:</h4>
<ul>
  <li><a href='https://github.com/RIOT-OS/RIOT/pull/14025.patch'>https://github.com/RIOT-OS/RIOT/pull/14025.patch</a></li>
  <li><a href='https://github.com/RIOT-OS/RIOT/pull/14025.diff'>https://github.com/RIOT-OS/RIOT/pull/14025.diff</a></li>
</ul>

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