Switch mtd-mac-address to nvmem implementation

[Ansuel Smith]

> > With a quick search, there are LOTS of dts that use these special bindings
> > and it would take a lot of time to convert all the dts. (think the best approach
> > would be to convert the dts one target at time)
>
> I think, that if you're willing to do that tedious hardwork, it's better to
> tackle all this at once. Prepare the changes, ask for testing and once we've
> few Tested-by: across popular targets it should be ready to merge.
>
> Thanks!
>
> Cheers,
>
> Petr

I posted on github a pr about this. (think it's would be easier to
review the changes there
since 99% of the changes are just sed command to change the old binding)
If anyone wants to check and give some opinion about the changes, feel
free to add any
comments.