MAINTAINERS: Add NS2 entry

Add NS2 to the Broadcom iProc Subsystem maintainers entry.  Since most
of the NS2 entries are already covered via the ns* already present
there, all that is currently needed is to reference the device tree
files.

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