dt-bindings: ARM: add arm,cortex-a72 compatible string

Signed-off-by: Masahiro Yamada <[e-mail]>
Signed-off-by: Rob Herring <[e-mail]>

Documentation/devicetree/bindings/arm/cpus.txt

···
 			    "arm,cortex-a17"
 			    "arm,cortex-a53"
 			    "arm,cortex-a57"
+			    "arm,cortex-a72"
 			    "arm,cortex-m0"
 			    "arm,cortex-m0+"
 			    "arm,cortex-m1"