Bugs in MMIO

[PeterRugg]

In our CHERI adaptation of Toooba, we identified a couple of bugs in MMIO that cause incorrect behaviour when accessing uncached devices. Unfortunately, the fixes aren't directly applicable to this repo because we are using our own AXI library, but I thought I'd report them here in case they trouble anybody else:

• Instruction bytes go missing (e.g. from bootrom). (Fixed by CTSRD-CHERI@44e19af which should be applicable). Depending on alignment of fetches from uncached instruction memory, some bytes would be shifted out.
• MMIO issues unaligned requests (Fixed by CTSRD-CHERI@cedb4c2 which isn't directly applicable). AXI flits had their widths hardwired to 8 bytes, even when accessing a small register within a memory mapped peripheral, causing unaligned requests that might confuse some peripherals.