WASHINGTON, Nov 26 (Reuters) - U.S. President Donald Trump said in an interview aired on Tuesday that he will designate Mexican drug cartels as terrorists over their role in drug and human trafficking. "They will be designated ... I have been working on that for the last 90 days. You know, designation is not that easy, you have to go through a process, and we are well into that process," Trump said in an interview with conservative media personality Bill O'Reilly that aired on Tuesday. Once a group is designated as a terrorist organization, under U.S. law it is illegal...