feat(mbedtls-psa): introduce PSA_CRYPTO build option

This is a preparatory patch to provide MbedTLS PSA Crypto
API support, with below changes -

1. Added a build macro PSA_CRYPTO to enable the MbedTLS PSA
   Crypto API support in the subsequent patches.
2. Compile necessary PSA crypto files from MbedTLS source code
   when PSA_CRYPTO=1.

Also, marked PSA_CRYPTO as an experimental feature.

Change-Id: I45188f56c5c98b169b2e21e365150b1825c6c450
Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
diff --git a/docs/getting_started/build-options.rst b/docs/getting_started/build-options.rst
index 7c84ef1..34d83f2 100644
--- a/docs/getting_started/build-options.rst
+++ b/docs/getting_started/build-options.rst
@@ -1185,6 +1185,12 @@
   errata mitigation for platforms with a non-arm interconnect using the errata
   ABI. By default its disabled (``0``).
 
+- ``PSA_CRYPTO``: Boolean option for enabling MbedTLS PSA crypto APIs support.
+  The platform will use PSA compliant Crypto APIs during authentication and
+  image measurement process by enabling this option. It uses APIs defined as
+  per the `PSA Crypto API specification`_. This feature is only supported if
+  using MbedTLS 3.x version. By default it is disabled (``0``).
+
 GICv3 driver options
 --------------------
 
@@ -1306,3 +1312,4 @@
 .. _GCC: https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html
 .. _Clang: https://clang.llvm.org/docs/DiagnosticsReference.html
 .. _Firmware Handoff specification: https://github.com/FirmwareHandoff/firmware_handoff/releases/tag/v0.9
+.. _PSA Crypto API specification: https://armmbed.github.io/mbed-crypto/html/