arm: octeontx2: Add Octeon TX2 CN913x DB support

This patch adds the base support for the Marvell Octeon TX2 CN913x DB.
Only one defconfig is added with this patch. Other board variants are
available (NAND, MMC booting) and images for these boards can be
generated by following the documentation added in the included README.

Signed-off-by: Konstantin Porotchkin <kostap@marvell.com>
Signed-off-by: Stefan Roese <sr@denx.de>