samples: nrf_cloud_multi_service: Fix MQTT with nRF7002DK

Two fixes: switch back to legacy crypto API for RSA, and switch
to TFM_IPC to circumvent a bug that caused TLS handshake failuire.

Signed-off-by: Simon Duquennoy <simon.duquennoy@nordicsemi.no>

Author: simonduq

samples/cellular/nrf_cloud_multi_service/overlay_nrf700x_wifi_mqtt_no_lte.conf

@@ -43,11 +43,11 @@ CONFIG_TFM_PARTITION_PROTECTED_STORAGE=y
 ## Configure TFM Profile. The NOT_SET profile will enable all features.
 ## We then reduce some settings to save flash and RAM.
 CONFIG_TFM_PROFILE_TYPE_NOT_SET=y
-## Select SFN for faster crypto
-CONFIG_TFM_IPC=n
-CONFIG_TFM_SFN=y
-CONFIG_TFM_CRYPTO_CONC_OPER_NUM=4
-CONFIG_TFM_CRYPTO_ASYM_SIGN_MODULE_ENABLED=n
+## Use IPC mode (default) — SFN mode has a bug that causes
+## psa_generate_key to fail during ECDHE key exchange.
+CONFIG_TFM_CRYPTO_ASYM_SIGN_MODULE_ENABLED=y
+## Enable MM-IOVEC to save RAM
+CONFIG_TFM_PSA_FRAMEWORK_HAS_MM_IOVEC=y
 
 ## Configure TFM partitions
 CONFIG_PM_PARTITION_SIZE_TFM_INTERNAL_TRUSTED_STORAGE=0x2000
@@ -140,10 +140,14 @@ CONFIG_MBEDTLS_ENABLE_HEAP=y
 CONFIG_MBEDTLS_RSA_C=y
 CONFIG_MBEDTLS_SSL_SERVER_NAME_INDICATION=y
 CONFIG_MBEDTLS_HEAP_SIZE=80000
+# Legacy API is required for RSA
+CONFIG_MBEDTLS_LEGACY_CRYPTO_C=y
+# ECDH/ECDSA are needed for TLS handshake
+CONFIG_PSA_WANT_ALG_ECDH=y
+CONFIG_PSA_WANT_ALG_ECDSA=y
 
 ## Disable unneeded MBEDTLS features to save flash and RAM
 CONFIG_MBEDTLS_CHACHA20_C=n
-CONFIG_MBEDTLS_CHACHA20_C=n
 CONFIG_MBEDTLS_CHACHAPOLY_C=n
 CONFIG_MBEDTLS_POLY1305_C=n
 CONFIG_MBEDTLS_SHA1_C=n