drivers: eth: Get Manual MAC address from devicetree

Move from a Kconfig to select/initialize the MAC address to using the
"local-mac-address" property in devicetree.  If the property is set the
drivers will initialize the mac-address from the devicetree (unless the
mac address is all 0's).  The MAC address might get overwritten by
either a driver specific means or by the setting of
"zephyr,random-mac-address" in the devicetree.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>