board: ns3: add support for Broadcom Northstar 3

Add support for Broadcom Northstar 3 SoC.
NS3 is a octo-core 64-bit ARMv8 Cortex-A72 processors
targeting a broad range of networking applications.

Signed-off-by: Rayagonda Kokatanur <rayagonda.kokatanur@broadcom.com>
Reviewed-by: Simon Glass <sjg@chromium.org>