net/fec: add mac field into platform data and consolidate fec_get_mac

Add mac field into fec_platform_data and consolidate function
fec_get_mac to get mac address in following order.

 1) module parameter via kernel command line fec.macaddr=0x00,0x04,...
 2) from flash in case of CONFIG_M5272 or fec_platform_data mac
    field for others, which typically have mac stored in fuse
 3) fec mac address registers set by bootloader

Signed-off-by: Shawn Guo <shawn.guo@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>