bsp: Add Corstone SSE-315 platform

* Add Corstone-315 platform to FreeRTOS Plus TCP
  Corstone-315 has got a different memory map than the
  previous Corstone platforms.
* Turn on BL1 logging for Corstone-315
* Don't define PLATFORM_DEFAULT_CRYPTO_KEYS
  for Corstone-315, this platform using its
  own keys, because of the new security IPs

Signed-off-by: Gabor Abonyi <gabor.abonyi@arm.com>