add support to read mac address for AM18x EVM manufactured from
Spectrum digital which have mac address stored in I2C EEPROM manufactured
by WINBOND. This patch reads mac address from I2C EEPROM and sets the
environment variable if not set, If mac address is already present in
environment compare it with the mac address in EEPROM and warn on
mismatch.Introduced a config option CONFIG_MAC_ADDR_IN_EEPROM to where
to look for the mac address.

Signed-off-by: Manjunath Hadli <manjunath.hadli@ti.com>
Cc: Tom Rini <trini@ti.com>
Cc: Mike Frysinger <vapier@gentoo.org>
Cc: Wolfgang Denk <wd@denx.de>