dt-bindings: Add a support cpu type for cortex-a73

Add arm cpu type cortex-a73
Signed-off-by: Chen Feng <puck.chen@hisilicon.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Wei Xu <xuwei5@hisilicon.com>

Documentation/devicetree/bindings/arm/cpus.txt

@@ -158,6 +158,7 @@ nodes to be present and contain the properties described below.
 			    "arm,cortex-a53"
 			    "arm,cortex-a57"
 			    "arm,cortex-a72"
+			    "arm,cortex-a73"
 			    "arm,cortex-m0"
 			    "arm,cortex-m0+"
 			    "arm,cortex-m1"