board: ns2: Add support for Broadcom Northstar 2

Add support for the Broadcom Northstar2 SoC and SVK (bcm958712k).  The
BCM5871X is a series of quad-core 64-bit 2GHz ARMv8 Cortex-A57
processors targeting a broad range of networking applications.

Signed-off-by: Jon Mason <jon.mason@broadcom.com>