dt-bindings: arm: add SMP enable-method for BCM2836

This adds a compatible string "brcm,bcm2836-smp" for Broadcom's
BCM2836 and BCM2837 to the 32 bit ARM CPU device tree binding.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Acked-by: Rob Herring <robh@kernel.org>