convert mtd-mac-address to nvmem implementation

[Ansuel Smith]

The dts has been migrated with a script and we decided to remove the
dtsi that caused some compilation problem.
We still need to take a decision for the device that have dtsi that
declare mtd-mac-address using partition tag declared in user dts
(dtsi use partition declared in the dts)

> I don't see a problem. I did check the dtsi files of the two routers which I'm using. One is migrated and the other one was still missing.