Add Secure Monitor

Adds a Secure Monitor to switch from non-secure EL1/2 to secure EL1
and back again.

* Passes information from BL2 to BL31 via BL1 on reentry to secure EL1
* Adds a dummy reciever of calls in BL2
* Adds functions to dump gp_regs and core_ctx
* Calls for functions not part of STD Service class are forwarded to
  secure EL1.