Feature request: GSU: add FastROM menu option

[hungLink]

This commit in the mister codebase allows for the fastROM option to be toggled: https://github.com/MiSTer-devel/SNES_MiSTer/commits/2408390003f49c7098d3a70e7a5e7950dd3a2663/SNES.sv

I thought the change might go in this file, but I have no idea what I'm doing. https://github.com/agg23/openfpga-SNES/blob/master/src/fpga/core/rtl/mister_top/SNES.sv

I found this commit as a resolution to this thread: https://github.com/MiSTer-devel/SNES_MiSTer/issues/320