bsp: Add Corstone-315 support

The files in the corstone315 folder come from the
SSE_315_BSP Keil pack.
https://www.keil.arm.com/packs/sse_315_bsp-arm/boards/



CMake support is added to access CMSIS Drivers and low level
peripheral drivers.

Signed-off-by: Dávid Házi <david.hazi@arm.com>
Signed-off-by: Gabor Abonyi <gabor.abonyi@arm.com>