Add PSA overview

Author: Alexander Zilberkant

docs/reference/api/security/security.md

@@ -1,13 +1,15 @@
 ## Security overview
 
-Security on Arm Mbed OS is divided into two parts:
+Security on Arm Mbed OS is divided into the following parts:
 
-- [Connection security through Arm Mbed TLS](/docs/development/reference/tls.html).
-- [Device security through Arm Mbed uVisor](/docs/development/reference/uvisor.html).
+- [Mbed TLS full documentation](https://tls.mbed.org/).
+- [Platform Security Architecture (PSA)](https://developer.arm.com/products/architecture/security-architectures/platform-security-architecture).
+- [Mbed uVisor full documentation](https://docs.mbed.com/docs/uvisor-and-uvisor-lib-documentation/en/latest/).
 
 <span class="warnings">**Warning**: uVisor is superseded by the Secure Partition Manager (SPM) defined in the ARM Platform Security Architecture (PSA). uVisor is deprecated as of Mbed OS 5.10, and being replaced by a native PSA-compliant implementation of SPM.</span>
 
-The sections cover working with these modules in the context of Mbed OS. For generic documentation for both modules, see:
+For working with these modules in Mbed OS context refer to the following links:
 
-- [Mbed TLS full documentation](https://tls.mbed.org/).
-- [Mbed uVisor full documentation](https://docs.mbed.com/docs/uvisor-and-uvisor-lib-documentation/en/latest/).
+- [Connection security through Arm Mbed TLS](/docs/development/reference/tls.html).
+- Platform Security Architecture - TBD - under development
+- [Device security through Arm Mbed uVisor](/docs/development/reference/uvisor.html).