PS/2 lacks any standard defining exact electrical and data protocol which leads to difficulties with some devices failing to work when connected together.
On the other hand, in contrast to USB HID protocol the PS/2 protocol has no practical limit on number of simultaneously pressed keys (full NKRO). This is important for special applications like computer stenography or hardcore gaming.
Driver for sunxi exists but is not integrated neither in sunxi-3.4 nor mainline kernel so far.
If you have a devboard that happens to have the PS/2 pins exposed on a header adding support should not be difficult.