net: enetc: fetch MAC address from device tree

Normally, the bootloader will already initialize the MAC address
registers of the ENETC and the driver will just use them or generate a
random one, if it is not initialized.

Add a new way to provide the MAC address: via device tree. Besides the
usual 'mac-address' property, there is also the possibility to fetch it
via a NVMEM provider. The sl28 board stores the MAC address in the SPI
NOR flash OTP region. Having this will allow linux to fetch the MAC
address from there without being dependent on the bootloader.

No in-tree boards have the device tree properties set, thus for these,
this is a no-op.

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>