Mmio<T> UnsafeCell

[todo]

Figure out if Mmio<T> should implement UnsafeCell. Does this mean that, just like atomic, write can take self by const reference only ? But is a Mmio<T> actually atomic ?
Forward all these questions to @roblabla.
Note:
see volatile cell and this discussion

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https://github.com/roblabla42/KFS/blob/c0c377d444ebf19b1143a9b835b4ff9cbf44f036/libutils/src/io/mmio.rs#L42-L52

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This issue was generated by todo based on a todo comment in c0c377d when #198 was merged. cc @Orycterope.