Add support for reading and writing mac addresses to or from ID EEPROM.

Added code for reading and writing Mac addresses to/from ID EEPROM(0x57).
With attached patch, we can use command "mac/mac read/mac save/"
to read and write EEPROM under u-boot prompt.

U-boot will calculate the checksum of EEPROM while bootup,
if it is right, then u-boot will check whether the mac address
of eTSEC0/1/2/3 is availalbe (non-zero).

If there is mac address availabe in EEPROM, u-boot will use it,
otherewise, u-boot will use the mac address defined in
MPC8641HPCN.h. This matches the requirement to set unique mac address
for each TSEC port.

Signed-off-by: Haiying Wang <haiying.wang@freescale.com>
Signed-off-by: York Sun <yorksun@freescale.com>