arm: pxa: init ethaddr for LP-8x4x using DT

When DT define aliases for etherner0 and ethernet1, U-Boot
automatically patched MAC addresses using ethaddr and eth1addr
environment variables respectively.

Custom initialization is no longer needed.

Signed-off-by: Sergei Ianovich <ynvich@gmail.com>
CC: Marek Vasut <marex@denx.de>