I'm in the US and from my experience Americans don't use the word porridge. Either the actual name of the grain -or a variant- is used (eg. oatmeal,hominy grits, cornmeal mush, cream of wheat/rice, etc.)or it's simply referred to as hot/cooked cereal. The only time I hear the wordporridge used it is by my West Indian acquaintances.

My understanding is that it's a general term for any type of boiled-until-mushy grainor legume (in the Bible Esau trades his birthright for a pottageof lentils -- porridge comes from the word pottage).