arm64: dts: move ns2 into northstar2 directory

Place northstar2 into its own subdirectory.  This helps as the number
of Broadcom boards grow and we can separate them per SoC.

Signed-off-by: Scott Branden <scott.branden@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>