AXI-in-simulation, DMA window fixes

[theturboturnip]

• Bump cheri-bgas-fuse-devfs to include correct WSTRB bits on 128-bit accesses, and to only expose a single H2F fmem device encompassing the entire window
• Add comments to explain the operation and memory map of the mkH2FAddrCtrl, which controls the DMA window
• Restrict the mkH2FAddrCtrl to return an address of Wd_Addr size, reflecting the intent and improving readability
• Correct the mkH2FAddrCtrl memory map - previously the first word of the DMA window address was presented at 0x0, then the second word at 0x1 regardless of the word width. Now if e.g. the H2F_lw words are 32-bit, the first word is mapped to 0x0 and the second to 0x4. This enables tinyemu-virtio to use 64-bit offsets, although I haven't tested that functionality yet.

[theturboturnip]

Closing in favor of #5 which is based on a branch inside this repo, not a fork