dt-bindings: doc: reflect new NVMEM of_get_mac_address behaviour

As of_get_mac_address now supports NVMEM under the hood, we need to update
the bindings documentation with the new nvmem-cell* properties, which would
mean copy&pasting a lot of redundant information to every binding
documentation currently referencing some of the MAC address properties.

So I've just removed all the references to the optional MAC address
properties and replaced them with the small note referencing
net/ethernet.txt file.

Signed-off-by: Petr Štetiar <ynezz@true.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>