net: mvneta.c: Add support for the ethernet controller of the Marvell Armada XP SoC

This patch adds support for the NETA ethernet controller which is integrated
in the Marvell Armada XP SoC's. This port is based on the Linux driver which
has been stripped of the in U-Boot unused portions.

Tested on the Marvell MV78460 eval board db-78460-bp.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Joe Hershberger <joe.hershberger@gmail.com>
Tested-by: Luka Perkov <luka@openwrt.org>