docs: update PSCI reference

PSCI specification reference in the documentation is updated
to point to latest specification and duplicate PSCI references are
removed.

Change-Id: I35ee365f08c557f3017af4d51f6d063a7501b27e
Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
diff --git a/docs/index.rst b/docs/index.rst
index bce9bb7..a7a5993 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -90,7 +90,7 @@
 
 .. _Armv7-A and Armv8-A: https://developer.arm.com/products/architecture/a-profile
 .. _Secure Monitor: http://www.arm.com/products/processors/technologies/trustzone/tee-smc.php
-.. _Power State Coordination Interface (PSCI): http://infocenter.arm.com/help/topic/com.arm.doc.den0022d/Power_State_Coordination_Interface_PDD_v1_1_DEN0022D.pdf
+.. _Power State Coordination Interface (PSCI): https://developer.arm.com/documentation/den0022/latest/
 .. _Trusted Board Boot Requirements CLIENT (TBBR-CLIENT): https://developer.arm.com/docs/den0006/latest/trusted-board-boot-requirements-client-tbbr-client-armv8-a
 .. _System Control and Management Interface (SCMI): http://infocenter.arm.com/help/topic/com.arm.doc.den0056a/DEN0056A_System_Control_and_Management_Interface.pdf
 .. _Software Delegated Exception Interface (SDEI): http://infocenter.arm.com/help/topic/com.arm.doc.den0054a/ARM_DEN0054A_Software_Delegated_Exception_Interface.pdf