fix(dmac): Use inline asm for compiler fences in DMAC

[jbeaurivage]

Summary

According to this discussion, it seems core::sync::atomic::{fence, compiler_fence} aren't a strong guarantee that the compiler won't reorder memory accesses beyond the fence. The suggested fix is to add an inline assembly call, which apparently guarantees there won't be any memory accesses beyond the asm call.

[rnd-ash]

Looks good @jbeaurivage , but maybe, its best to put the fence functions near the crate root for future use by other peripherals should we need to in future? - Or is this specific to DMAC?