Add custom save type

[Mr-Wiseguy]

This save type has a very simple interface with two functions (shown from the perspective of recompiled code):

• void recomp_save_write(void* rdram_address, u32 offset, u32 count);
• void recomp_save_read(void* rdram_address, u32 offset, u32 count);
  It can be used when the save type is Custom or AllowAll and has a configurable size that's set in the GameEntry.