dt-bindings: arm: add cortex-a12 and cortex-a17 cpu compatible properties

As announced parts from ARM they will probably be used in socs shortly.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>