hw/arm/bcm2836: Use the Cortex-A7 instead of Cortex-A15

hw/arm/bcm2836: Use the Cortex-A7 instead of Cortex-A15

The BCM2836 uses a Cortex-A7, not a Cortex-A15. Update the device to
use the correct CPU.
https://www.raspberrypi.org/documentation/hardware/raspberrypi/bcm2836/QA7_rev3.4.pdf

When the BCM2836 was introduced (bad5623690b) the Cortex-A7 was not
available, so the very similar Cortex-A15 was used. Since dcf578ed8ce
we can model the correct core.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alistair Francis <alistair@alistair23.me>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20180319110215.16755-1-peter.maydell@linaro.org