dt-bindings: arm: Add entry for Broadcom Brahma-B53

Broadcom's Brahma-B53 CPU is an ARMv8A processor used on a number of
DSL, Cable Modem and Set-top-box SoCs.

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