xilinx: board: Use ETH_ALEN macro for mac address size

Use predefined macro for eth_mac legacy format.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/382d4bff370894164fad3c30ac0400b25142b544.1674573561.git.michal.simek@amd.com