ARM: brcmstb: add CPU binding for Broadcom Brahma15

Add the Broadcom Brahma B15 CPU to the DT CPU binding list.

Signed-off-by: Marc Carino <marc.ceeeee@gmail.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Matt Porter <mporter@linaro.org>