Because the PSP hardware was discontinued long before this game was developed, there is no official "PSP file" or ISO for Ragnarök. If you see sites offering a "God of War Ragnarök PSP download," be very careful—these are often scams or malware.
Any "Ragnarök" files found for PSP or the PPSSPP emulator are typically fan-made mods of older titles like Ghost of Sparta Chains of Olympus god of war ragnarok psp file download best