Switch mtd-mac-address to nvmem implementation

[Ansuel Smith]

The pending nvmem patches have been applied to mtd/next.
Some time ago I also sent a patch with the mtd-mac-address-increment-byte
upstream but it got rejected since dts should not contain this kind of binding.

The patch directly implements the increment and increment-byte binding
to the of_get_mac_address.

So now the question, to clean things and follow a more standard framework,
wouldn't be better to replace the patch generic pending/681 with a more
generic implementation and convert all the dts to the standard mac-address
binding and nvmem implementation?

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)

To improve this, I would propose to modify the 681 patch to only add
the mtd-mac-address binding and the reading of the mac address from mtd and
take my patch that implement the increment and increment-byte function to the
general of_get_mac_address.

This way the hard part of defining the nvmem cell can be postponed and the
mtd-mac-address-increment-byte and mtd-mac-address-increment can simply
be renamed to the new more generic mac-address-increment-byte and
mac-address-increment binding with no regression since they implement the
same thing in the same way.