ARM: bcm281xx: Add DT support for SMC handler

- Adds devicetree binding and documentation for the smc handler

Updates from V1:
- Created this separate patch for the DT portion
- Added SoC compatible to smc binding

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