Add power management support in the SPD ...

This patch implements a set of handlers in the SPD which are called by
the PSCI runtime service upon receiving a power management
operation. These handlers in turn pass control to the Secure Payload
image if required before returning control to PSCI. This ensures that
the Secure Payload has complete visibility of all power transitions in
the system and can prepare accordingly.

Change-Id: I2d1dba5629b7cf2d53999d39fe807dfcf3f62fe2

Add support for handling runtime service requests ...

This patch uses the reworked exception handling support to handle
runtime service requests through SMCs following the SMC calling
convention. This is a giant commit since all the changes are
inter-related. It does the following:

1. Replace the old exception handling mechanism with the new one
2. Enforce that SP_EL0 is used C runtime stacks.
3. Ensures that the cold and warm boot paths use the 'cpu_context'
   structure to program an ERET into the next lower EL.
4. Ensures that SP_EL3 always points to the next 'cpu_context'
   structure prior to an ERET into the next lower EL
5. Introduces a PSCI SMC handler which completes the use of PSCI as a
   runtime service

Change-Id: I661797f834c0803d2c674d20f504df1b04c2b852
Co-authored-by: Achin Gupta <achin.gupta@arm.com>

Add context management library ...

This patch adds support for a cpu context management library. This
library will be used to:

1. Share pointers to secure and non-secure state cpu contexts between
   runtime services e.g. PSCI and Secure Payload Dispatcher services
2. Set SP_EL3 to a context structure which will be used for
   programming an ERET into a lower EL
3. Provide wrapper functions to save and restore EL3 & EL1
   state. These functions will in turn use the helper functions in
   context.S

Change-Id: I655eeef83dcd2a0c6f2eb2ac23efab866ac83ca0