Add Cortex-A15 CPU definition

Add Cortex-A15 CPU definition

Add a definition of a Cortex-A15 CPU. Note that for the moment we do
not implement any of:
 * Large Physical Address Extensions (LPAE)
 * Virtualization Extensions
 * Generic Timer
 * TrustZone (this is also true of our existing Cortex-A9 model, etc)

This CPU model is sufficient to boot a Linux kernel which has been
compiled for an A15 without LPAE enabled.

Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>