arm64: versal-net: Add defconfig for Versal NET

Use one defconfig for supporting multiple different platforms. DTB
reselection is enabled to choose DT based on SOC detection.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/82cc7c1ca8850270cc2ebc992d835a37aa3d236f.1663589964.git.michal.simek@amd.com