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This patch updates the relevant documentation in ARM Trusted Firmware
for the new GIC drivers. The user-guide.md and porting-guide.md have been
updated as follows:

*  The build option to compile Trusted Firmware with different GIC drivers
   for FVP has been explained in the user-guide.md.

*  The implementation details of interrupt management framework porting
   APIs for GICv3 have been added in porting-guide.md.

*  The Linaro tracking kernel release does not work OOB in GICv3 mode.
   The instructions for changing UEFI configuration in order to run with
   the new GICv3 driver in ARM TF have been added to user-guide.md.

The interrupt-framework-design.md has been updated as follows:

*  Describes support for registering and handling interrupts targeted to EL3
   e.g. Group 0 interrupts in GICv3.

*  Describes the build option `TSP_NS_INTR_ASYNC_PREEMPT` in detail.

*  Describes preemption of TSP in S-EL1 by non secure interrupts and
   also possibly by higher priority EL3 interrupts.

*  Describes the normal world sequence for issuing `standard` SMC calls.

*  Modifies the document to correspond to the current state of interrupt
   handling in TSPD and TSP.

*  Modifies the various functions names in the document to reflect
   the current names used in code.

Change-Id: I78c9514b5be834f193405aad3c1752a4a9e27a6c