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: Ben Warren <biggerbadderben@gmail.com>
Acked-by: Detlev Zundel <dzu@denx.de>
Tested-by: Prafulla Wadaskar <prafulla@marvell.com>
Tested-by: Heiko Schocher <hs@denx.de>
Tested-by: Thomas Chou <thomas@wytron.com.tw>

As this seems unclear, document how the flow of setting up
the MAC address is correct.

Signed-off-by: Heiko Schocher <hs@denx.de>

Text changed slightly, adding input from Mike Frysinger.

Signed-off-by: Wolfgang Denk <wd@denx.de>

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
CC: Ben Warren <biggerbadderben@gmail.com>