dt-bindings: Add new SoCs to bcm4708 DT bindings

Add the 4708, 4709, and 53012 SoCs to the the documentation for the
Broadcom Northstar device tree bindings.

Signed-off-by: Jon Mason <jonmason@broadcom.com>
Acked-by: Hauke Mehrtens <hauke@hauke-m.de>
Acked-by: Scott Branden <sbranden@broadcom.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>