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>