Add a new function to the eth_device struct for programming a network
controller's hardware address.

After all network devices have been initialized and the proper MAC address
for each has been determined, make a device driver call to program the
address into the device.  Only device instances with valid unicast addresses
will be programmed.
Signed-off-by: NBen Warren <biggerbadderben@gmail.com>
Acked-by: NDetlev Zundel <dzu@denx.de>
Tested-by: NPrafulla Wadaskar <prafulla@marvell.com>
Tested-by: NHeiko Schocher <hs@denx.de>
Tested-by: NThomas Chou <thomas@wytron.com.tw>