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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>

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