Naming of cv.slet

Created by: MaryBennett

Make the change: https://github.com/openhwgroup/cv32e40p/issues/833

I suggest:

1. Rename cv.slet to cv.sle.
2. Change cv.slet to an instruction alias for cv.sle.
3. Repeat for cv.sletu -> cv.sleu.

Similar issues: https://github.com/openhwgroup/corev-binutils-gdb/issues/100