feat(plat/arm/css): add per-cpu power down support for warm reset

Add a new function to setup a SGI interrupt that will be used to trigger
a request for per-cpu power down when executing the PSCI SYSTEM_RESET
request. This will be used on CSS platform that require all the CPUs to
execute the CPU specific power down sequence to complete a warm reboot
sequence in which only the CPUs are power cycled.

Change-Id: I80da0f6c3cd0c5c442c82239ba1e1f773821a7f5
Signed-off-by: Pranav Madhu <pranav.madhu@arm.com>
diff --git a/docs/plat/arm/arm-build-options.rst b/docs/plat/arm/arm-build-options.rst
index 339ebbe..afe89b9 100644
--- a/docs/plat/arm/arm-build-options.rst
+++ b/docs/plat/arm/arm-build-options.rst
@@ -152,6 +152,11 @@
     to select the appropriate platform variant for the build. The range of
     valid values is platform specific.
 
+- ``CSS_SYSTEM_GRACEFUL_RESET``: Build option to enable graceful powerdown of
+   CPU core on reset. This build option can be used on CSS platforms that
+   require all the CPUs to execute the CPU specific power down sequence to
+   complete a warm reboot sequence in which only the CPUs are power cycled.
+
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 .. |FIP in a GPT image| image:: ../../resources/diagrams/FIP_in_a_GPT_image.png