SPMD: add SPM dispatcher based upon SPCI Beta 0 spec

This patch adds a rudimentary SPM dispatcher component in EL3.
It does the following:

- Consumes the TOS_FW_CONFIG to determine properties of the SPM core
  component
- Initialises the SPM core component which resides in the BL32 image
- Implements a handler for SPCI calls from either security state. Some
  basic validation is done for each call but in most cases it is simply
  forwarded as-is to the "other" security state.

Signed-off-by: Achin Gupta <achin.gupta@arm.com>
Signed-off-by: Artsem Artsemenka <artsem.artsemenka@arm.com>
Change-Id: I7d116814557f7255f4f4ebb797d1619d4fbab590