<h3>Contribution description</h3>
<p>Similar to <a class="issue-link js-issue-link" data-error-text="Failed to load issue title" data-id="493784232" data-permission-text="Issue title is private" data-url="https://github.com/RIOT-OS/RIOT/issues/12236" data-hovercard-type="pull_request" data-hovercard-url="/RIOT-OS/RIOT/pull/12236/hovercard" href="https://github.com/RIOT-OS/RIOT/pull/12236">#12236</a>, this enables the pullup resistor on the UART RX pin. This is to fix the undefined behavior that results when the RX pin is floating.</p>

<h3>Testing procedure</h3>
<p>Confirm that a UART shell still works as intended.</p>
<p>To test that this PR effects a beneficial change, use the test in <a class="issue-link js-issue-link" data-error-text="Failed to load issue title" data-id="493784232" data-permission-text="Issue title is private" data-url="https://github.com/RIOT-OS/RIOT/issues/12236" data-hovercard-type="pull_request" data-hovercard-url="/RIOT-OS/RIOT/pull/12236/hovercard?comment_id=531613204&comment_type=issue_comment" href="https://github.com/RIOT-OS/RIOT/pull/12236#issuecomment-531613204">#12236 (comment)</a>. In fact the results I posted there were from an efm32 board.</p>

<h3>Issues/PRs references</h3>
<p>This does for efm32 what <a class="issue-link js-issue-link" data-error-text="Failed to load issue title" data-id="493784232" data-permission-text="Issue title is private" data-url="https://github.com/RIOT-OS/RIOT/issues/12236" data-hovercard-type="pull_request" data-hovercard-url="/RIOT-OS/RIOT/pull/12236/hovercard" href="https://github.com/RIOT-OS/RIOT/pull/12236">#12236</a> did for stm32.</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/12279'>https://github.com/RIOT-OS/RIOT/pull/12279</a></p>

<h4>Commit Summary</h4>
<ul>
  <li>cpu/efm32/uart: enable pullup on RX pin</li>
</ul>

<h4>File Changes</h4>
<ul>
  <li>
    <strong>M</strong>
    <a href="https://github.com/RIOT-OS/RIOT/pull/12279/files#diff-0">cpu/efm32/periph/uart.c</a>
    (2)
  </li>
</ul>

<h4>Patch Links:</h4>
<ul>
  <li><a href='https://github.com/RIOT-OS/RIOT/pull/12279.patch'>https://github.com/RIOT-OS/RIOT/pull/12279.patch</a></li>
  <li><a href='https://github.com/RIOT-OS/RIOT/pull/12279.diff'>https://github.com/RIOT-OS/RIOT/pull/12279.diff</a></li>
</ul>

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