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: NRob Herring <robh@kernel.org>
Signed-off-by: NFlorian Fainelli <f.fainelli@gmail.com>

Documentation/devicetree/bindings/arm/cpus.txt

@@ -169,6 +169,7 @@ described below.
 			    "arm,cortex-r5"
 			    "arm,cortex-r7"
 			    "brcm,brahma-b15"
+			    "brcm,brahma-b53"
 			    "brcm,vulcan"
 			    "cavium,thunder"
 			    "cavium,thunder2"