feat(xilinx): add handler for power down req sgi irq

On receiving CPU power down callback, TF-A raises SGI interrupt to all active
cores to power down each active cores. Add handler for this SGI IRQ.

By default TF-A uses SGI 6 for CPU power down request. This can be
configurable through CPU_PWRDWN_SGI build flag.

e.g., If user wants to use SGI 7 instead of SGI 6 then provide build
flag CPU_PWRDWN_SGI=7

Signed-off-by: Jay Buddhabhatti <jay.buddhabhatti@amd.com>
Change-Id: Id0df32187d1de3f0af4486eb4d4930cb3ab01dbd
diff --git a/docs/plat/xilinx-versal-net.rst b/docs/plat/xilinx-versal-net.rst
index 1db7695..3f31d40 100644
--- a/docs/plat/xilinx-versal-net.rst
+++ b/docs/plat/xilinx-versal-net.rst
@@ -40,3 +40,16 @@
 *   `TFA_NO_PM` : Platform Management support.
     -    0 : Enable Platform Management (Default)
     -    1 : Disable Platform Management
+
+*   `CPU_PWRDWN_SGI`: Select the SGI for triggering CPU power down request to
+                      secondary cores on receiving power down callback from
+                      firmware. Options:
+
+    -   `0`   : SGI 0
+    -   `1`   : SGI 1
+    -   `2`   : SGI 2
+    -   `3`   : SGI 3
+    -   `4`   : SGI 4
+    -   `5`   : SGI 5
+    -   `6`   : SGI 6 (Default)
+    -   `7`   : SGI 7