ARM: davinci: dm644x-evm: use cell nvmem lookup for mac address

We now support nvmem lookups and cell definitions for machine code.
Add relevant data structures for the mac-address stored in at24 EEPROM.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>