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

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>