dt-bindings: add SMP enable-method for Broadcom NSP

Add a compatible string "brcm,bcm-nsp-smp" for Broadcom's
Northstar Plus CPU to the 32-bit ARM CPU device tree binding
documentation file and create a new binding documentation for
Northstar Plus CPU.

Signed-off-by: Kapil Hali <kapilh@broadcom.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>