arm: add Cubieboard7 board support

The Cubieboard is a single board computer containing a
Actions S700 SoC(with 4 ARMv8 Cortex-A53 cores).

This patch adds respective defconfig alongwith .dts(copied
from Linux v5.5-rc6 with hash "b3a987b0264d").

Signed-off-by: Amit Singh Tomar <amittomer25@gmail.com>