I have a couple of papers that I have submitted to some journals but no feedback is received yet. meanwhile i'm writing my phd thesis and the defense is in few months, surely before getting any feedback from the journals. I'm wondering if it is okay to mention those submitted journal papers in my thesis.
Yes. It is both OK and common to mention them as Articles Submitted, or as Manuscripts Under Review, or in case you have any (e.g. arXiv) preprint, Article Preprints (with the appropriate arXiv identifier).
It might be relevant for me to point out from personal experience, that the only instruction from my department in this regard was - "Everything mentioned in the list of publications in the thesis has to be either published, or submitted for publication. There shouldn't be anything in the articles to be submitted category, because that category could be inflated as much as a person desires". The last part may be field and institute-dependent, but I think this conveys the basic ideology. Articles submitted are safe in terms of inclusion.
If a paper does not exist (outside your computer), it is not a reference and should not be cited. You should at least make them available online. Publishing at ArXiv is one approach.
It's not obligatory to mention papers in your dissertation, except if you take a lot of text from them (particularly if they are multi-authored) then you really should cite them or even put a footnote at the beginning of the chapter (if a lot or all of the material from a chapter comes from one more more papers) with the title and author list, and where published if it is published. But if you aren't the sole author, you should do this whether or not the paper is published.
So, it's obligatory to mention any help you've had with the material in your dissertation, and to cite yourself like anyone else if you reference your own work a great deal. But your dissertation is not your CV. If your examiners wonder about your papers, you can tell them. When you apply for jobs, you will be expecting them to read a list of papers on your CV, and not your entire dissertation. In general, you should expect people are looking at your Google Scholar page, academia.edu, home web page etc. But (sad to say) next to no one who has power over you will read your dissertation all the way through. Mostly that will be done by students who come after you.
I'd say, unless you have specific instructions from your university (or your committee, or your advisor), you should probably do the same thing as you would for published articles. If your thesis mentions the articles you've published, then it should also mention the articles you've submitted. Otherwise, it shouldn't.
Unless you're asking whether you should put those articles in a bibliography/list of cited references. In that case, follow the same rules you would follow for referencing an unpublished but submitted article which someone else had written.