Can you make porridge out of P.A.N. harina?
You cannot make grits, because by definition, true hominy grits are treated with alkali which is what makes them different than plain cornmeal or polenta. 

Searching for harina pan polenta indicates a variety of recipes are possible (most are in Spanish, so I cannot read them).  However, it doesn't appear that a porridge type dish is the usual application for this product.

Still, the nature of the product indicates it should work, although the result may be slightly different in texture and flavor from more traditional Italian style polenta, since the harina PAN is precooked before being dried and ground.   It should therefore cook up much faster than one made with regular corn meal, analogous to the difference between instant oatmeal and traditional outmeal.

There would be no single ratio of water to grain, however, as polenta can range from a thin porridge to a thick enough to cut into slices and fry.  You will have to experiment to find the ratio that you like the best.  I would suggest starting out at about 2:1 liquid to harina pan by volume.