I doubt there's a firmware, Canon intentionally didn't put in the incremental ISOs. You can try Magic Lantern if you really want the intermediate ISO. I'm not sure if there's a stable ML for the T5i or not though.
However, you may want to do some research on incremental ISO, it's not all it's cracked up to be. The +1/3 stop is actually just the full stop ISO (e.g. 100) pushed up a third stop (e.g. 125) via an amplifier, and is actually noisier than the next full stop (e.g. 200). It gets complicated from there... technically 160 is cleaner than 100, but you lose some DR on the top. It's similar to exposing to the right. I played around with it for awhile on my 6D, then decided to just set it to full stop increments and now worry about the hassle.
