of_net: Fix residues after of_get_nvmem_mac_address removal

I've discovered following discrepancy in the bindings/net/ethernet.txt
documentation, where it states following:

 - nvmem-cells: phandle, reference to an nvmem node for the MAC address;
 - nvmem-cell-names: string, should be "mac-address" if nvmem is to be..

which is actually misleading and confusing. There are only two ethernet
drivers in the tree, cadence/macb and davinci which supports this
properties.

This nvmem-cell* properties were introduced in commit 9217e566
("of_net: Implement of_get_nvmem_mac_address helper"), but
commit afa64a72 ("of: net: kill of_get_nvmem_mac_address()")
forget to properly clean up this parts.

So this patch fixes the documentation by moving the nvmem-cell*
properties at the appropriate places.  While at it, I've removed unused
include as well.

Cc: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Fixes: afa64a72 ("of: net: kill of_get_nvmem_mac_address()")
Signed-off-by: Petr Štetiar <ynezz@true.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>