SPEAr : Support for HW mac id read/write from i2c mem

This patch adds the  support to read and write mac id from i2c
memory.
For reading:
	if (env contains ethaddr)
		pick env ethaddr
	else
		pick ethaddr from i2c memory
For writing:
	chip_config ethaddr XX:XX:XX:XX:XX:XX writes the mac id
	in i2c memory

Signed-off-by: Vipin <vipin.kumar@st.com>