trusty: keymaster: Implement add_rng_entropy

Test: builds
Change-Id: Iec5f61777f9b39136028d61acaf724b9fafb4492
diff --git a/trusty/keymaster/trusty_keymaster_device.cpp b/trusty/keymaster/trusty_keymaster_device.cpp
index 5bc879c..c1c43f5 100644
--- a/trusty/keymaster/trusty_keymaster_device.cpp
+++ b/trusty/keymaster/trusty_keymaster_device.cpp
@@ -165,7 +165,15 @@
 
 keymaster_error_t TrustyKeymasterDevice::add_rng_entropy(const uint8_t* data, size_t data_length) {
     ALOGD("Device received add_rng_entropy");
-    return KM_ERROR_OK;
+
+    if (error_ != KM_ERROR_OK) {
+        return error_;
+    }
+
+    AddEntropyRequest request;
+    request.random_data.Reinitialize(data, data_length);
+    AddEntropyResponse response;
+    return Send(KM_ADD_RNG_ENTROPY, request, &response);
 }
 
 keymaster_error_t TrustyKeymasterDevice::generate_key(