memory descriptors does not expose save RAM

[Jamiras]

As noted in #87, the achievements code cannot find the save RAM

[INFO] [MEM] Registered 0x20000 bytes of SYSTEM RAM at $000000 (descriptor 5, offset 0x000000)
[INFO] [MEM] Could not map region starting at $020000
[INFO] [MEM] Registered 0x20000 bytes of SRAM at $020000 (null filler)

It's expecting to find a memory descriptor that contains the address $FE0000.