Add disambiguous entries

[kersten1]

Disambiguation entries, sort of like what Wikipedia does, for things which have the same acronym. For example you have IMSIC as one acronym, but not the one from AIA: Incoming MSI Controller, which in the RISC-V world is arguably more important. Also PLIC is Platform-Level Interrupt Controller. Also RV is sometimes an abbreviation for RISC-V.