Update Mbed TLS to 3.5.1

Commit ab5707185a9e ("Add a minimum rsa key size config to psa config")
introduced a minimal RSA key size condition to prevent accidental
misuse. The limit is set to 1024 bits and generating shorter RSA keys
will result in PSA_ERROR_NOT_SUPPORTED. Increase key size crypto tests
in TS service test to pass this check.

Signed-off-by: Imre Kis <imre.kis@arm.com>
Change-Id: I917d0bf572fbf763a68c6d479d23ba66e9da6c13
diff --git a/external/MbedTLS/MbedTLS.cmake b/external/MbedTLS/MbedTLS.cmake
index 4be70e6..fd83d01 100644
--- a/external/MbedTLS/MbedTLS.cmake
+++ b/external/MbedTLS/MbedTLS.cmake
@@ -7,7 +7,7 @@
 
 set(MBEDTLS_URL "https://github.com/Mbed-TLS/mbedtls.git"
 		CACHE STRING "Mbed TLS repository URL")
-set(MBEDTLS_REFSPEC "mbedtls-3.4.0"
+set(MBEDTLS_REFSPEC "mbedtls-3.5.1"
 		CACHE STRING "Mbed TLS git refspec")
 set(MBEDTLS_SOURCE_DIR "${CMAKE_CURRENT_BINARY_DIR}/_deps/mbedtls-src"
 		CACHE PATH "MbedTLS source directory")