crypto: Add mbedtls-psa as a submodule

mbedtls-psa contains an implementation of libmbedcrypto, including the PSA
Crypto API.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 06f897e..19ab4eb 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -9,6 +9,7 @@
 option(ENABLE_ZLIB_SUPPORT "Build mbed TLS with zlib library." OFF)
 
 option(ENABLE_PROGRAMS "Build mbed TLS programs." ON)
+option(USE_CRYPTO_SUBMODULE "Build and use libmbedcrypto from the crypto submodule." OFF)
 
 option(UNSAFE_BUILD "Allow unsafe builds. These builds ARE NOT SECURE." OFF)
 
@@ -177,6 +178,10 @@
 
 add_subdirectory(library)
 add_subdirectory(include)
+if(USE_CRYPTO_SUBMODULE)
+    add_subdirectory(crypto/library)
+    add_subdirectory(crypto/include)
+endif()
 
 if(ENABLE_PROGRAMS)
     add_subdirectory(programs)