Add support for mapper 473

[Masteretro]

info nesdev.org NES_2.0_Mapper_473
dump multirom in my repository:
500 IN 1 Classic [Mapper 473]

[keithbowes]

It's weird to make a game using a mapper that no core supports. I get a black screen in fceumm, a green screen in Mesen, and a graceful exit in Nestopia.