Add a wrapper function for psa_unregister_read

There are at least 20 occurences in the current code where
we will need this pattern of code, so I thought it best to
put this in a function

Signed-off-by: Ryan Everett <ryan.everett@arm.com>
diff --git a/library/psa_crypto_slot_management.c b/library/psa_crypto_slot_management.c
index fd33ad9..53ebf31 100644
--- a/library/psa_crypto_slot_management.c
+++ b/library/psa_crypto_slot_management.c
@@ -458,6 +458,21 @@
     return PSA_ERROR_CORRUPTION_DETECTED;
 }
 
+psa_status_t psa_unregister_read_under_mutex(psa_key_slot_t *slot)
+{
+    psa_status_t status = PSA_ERROR_CORRUPTION_DETECTED;
+#if defined(MBEDTLS_THREADING_C)
+    PSA_THREADING_CHK_RET(mbedtls_mutex_lock(
+                              &mbedtls_threading_key_slot_mutex));
+#endif
+    status = psa_unregister_read(slot);
+#if defined(MBEDTLS_THREADING_C)
+    PSA_THREADING_CHK_RET(mbedtls_mutex_unlock(
+                              &mbedtls_threading_key_slot_mutex));
+#endif
+    return status;
+}
+
 psa_status_t psa_validate_key_location(psa_key_lifetime_t lifetime,
                                        psa_se_drv_table_entry_t **p_drv)
 {