Autonomous random driver: declare the type and function

Define a sample type mbedtls_psa_external_random_context_t in
psa/crypto_platform.h and define the prototype of
mbedtls_psa_external_get_random() in a public header.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/include/psa/crypto_platform.h b/include/psa/crypto_platform.h
index 567398d..a147a3e 100644
--- a/include/psa/crypto_platform.h
+++ b/include/psa/crypto_platform.h
@@ -81,4 +81,10 @@
 
 #endif /* MBEDTLS_PSA_CRYPTO_KEY_ID_ENCODES_OWNER */
 
+#if defined(MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG)
+typedef struct {
+    uint8_t opaque[32];
+} mbedtls_psa_external_random_context_t;
+#endif /* MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG */
+
 #endif /* PSA_CRYPTO_PLATFORM_H */