How do I add a title attribute to the search submit button, exposed search, and reset buttons?
I have found the answer of my question.

I have used the following code in my form_alter function.

if ($form_id == 'search_block_form') {
   $form['actions']['submit']['#attributes']['title'] = t('Search');
  }
  if ($form_id == 'search_form') {
   $form['advanced']['submit']['#attributes']['title'] = t('Search');
  }
  if($form_id == 'views_exposed_form') {
    $form['submit']['#attributes']['title'] = t('Search');
   $form['reset']['#attributes']['title'] = t('Reset');
  }
  return $form;


It solved my problem.