arm64: dts: Move NS2 clock DT nodes to separate DT file

For more readabilty and consistency with other Broadcom SoCs, we move
all NS2 clock DT nodes from main SoC DT file to a separate DT file.

We also update the license header in ns2.dtsi as-per new Broadcom
convention.

Signed-off-by: Anup Patel <anup.patel@broadcom.com>
Reviewed-by: Ray Jui <rjui@broadcom.com>
Reviewed-by: Scott Branden <sbranden@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>