volatile struct definition for peripheral control registers

[Josh Holtrop]

Just to follow up, I introduced the `Volatile` struct to my 
project and had to add a couple extra functions to make 
everything happy:

```d
module myproj.volatile;

import core.volatile;

struct Volatile(T)
{
     private T value;

     public @property T read()
     {
         return volatileLoad(&value);
     }
     alias read this;

     public void opAssign(T value)
     {
         volatileStore(&this.value, value);
     }

     public T opUnary(string s)()
     {
         T v = read();
         mixin(s ~ "v;");
         volatileStore(&this.value, v);
         return v;
     }

     public T opOpAssign(string s)(T rhs)
     {
         T v = read();
         mixin("v " ~ s ~ "= rhs;");
         volatileStore(&this.value, v);
         return v;
     }
}
```

So far it is working great!