arm64: dts: Add Broadcom North Star 2 support

Add Broadcom NS2 device tree binding document. Also add initial device
tree dtsi for Broadcom North Star 2 (NS2) SoC and board support for NS2
SVK board

Signed-off-by: Jon Mason <jonmason@broadcom.com>
Signed-off-by: Ray Jui <rjui@broadcom.com>
Reviewed-by: Scott Branden <sbranden@broadcom.com>
Signed-off-by: Olof Johansson <olof@lixom.net>