psa: Add PSA_KEY_ID_INIT

A macro useful for initializing psa_key_id_t, whether
MBEDTLS_PSA_CRYPTO_KEY_FILE_ID_ENCODES_OWNER is set or not. Without this
macro, it is necessary to know if
MBEDTLS_PSA_CRYPTO_KEY_FILE_ID_ENCODES_OWNER as with it the key ID is
non-scalar and needs to be initialized with {0, 0}, and 0 otherwise when
key ID is scalar.
diff --git a/include/psa/crypto_platform.h b/include/psa/crypto_platform.h
index 86af08f..572f40c 100644
--- a/include/psa/crypto_platform.h
+++ b/include/psa/crypto_platform.h
@@ -89,6 +89,7 @@
  * `psa_key_file_id_t` argument. As a workaround, make `psa_key_id_t` an
  * alias for `psa_key_file_id_t` when building for a multi-client service. */
 typedef psa_key_file_id_t psa_key_id_t;
+#define PSA_KEY_ID_INIT {0, 0}
 
 #else /* !MBEDTLS_PSA_CRYPTO_KEY_FILE_ID_ENCODES_OWNER */