da850evm: add support to read mac address from SPI flash

add support to read mac address for da850/L138 evm manufactured by
Logic PD which store mac address in SPI flash manufactured by ST
Microelectronics. This patch adds support to read mac address from
SPI flash and set the mac address if it hasn't been set in environment,
If it has been set in the environment it compares the value one with
SPI flash and warns on mismatch. Introduced a config option
CONFIG_MAC_ADDR_IN_SPIFLASH indicating where to look mac address for.

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>