Trimming the fat and reintroducing gcn-sd

[techflashYT]

gcn-sd (the SDGecko driver for the Nintendo GameCube and Wii) was working "fine" via a tremendous amount of hacks, and duplicated code. This was up until 5.x, when the legacy request_fn block drivers got dropped in favor of blk-mq, at which point, gcn-sd was dropped.

Since EXI is quite literally just SPI but "stupidly fast" (according to JoJo Autoboy on Discord, and he has the hardware proof to back it up), it would be ideal to implement a layer such that EXI could be treated as a proper SPI controller, and then it would use the upstream mmc_spi driver wholesale, no special driver for SDGecko.
I'm not sure how much effort that would take, but it would be by far the best solution to this problem.

[techflashYT]

So I've managed to make a blk-mq version of gcn-sd (see the wii-v4.19-gcn-sd-mq branch), but it has weird data corruption issues that I've not yet identified. The standard 4.19 branch is still the only one with a fully working gcn-sd.