ARM: bcm281xx: Add L2 cache enable code

- Adds a module to provide calls into secure monitor mode
- Uses this module to make secure monitor calls to enable L2 cache.

Updates from V1:
- Split DT portion into separate patch
- replace #ifdef-ed L2 code with "if".
- move init call to board init

Signed-off-by: Christian Daudt <csd@broadcom.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>