net: Add support for reading mac addresses from nvmem cells

This adds support for reading mac addresses from the "mac-address" nvmem
cell. If there is no (local-)mac-address property, then we will try
reading from an nvmem cell.

For some existing examples of this property, refer to imx8mn.dtsi and
imx8mp.dtsi. Unfortunately, fuse drivers have not yet been converted
to DM.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Reviewed-by: Simon Glass <sjg@chromium.org>