trng: Move device memory retrieve to platform code

trng_adapter_init() was checking for device memory. However not every
trng implementation needs device memory. Instead we move the device
memory retrieval to the platform specific code.

Signed-off-by: Jelle Sels <jelle.sels@arm.com>
Change-Id: Ib4ebca98152900e471711a9106682cd43510057d
diff --git a/platform/drivers/mock/mock_trng.c b/platform/drivers/mock/mock_trng.c
index 24b14c0..a9f79f8 100644
--- a/platform/drivers/mock/mock_trng.c
+++ b/platform/drivers/mock/mock_trng.c
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2021, Arm Limited and Contributors. All rights reserved.
+ * Copyright (c) 2021-2022, Arm Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -24,13 +24,11 @@
     return 0;
 }
 
-int platform_trng_create(struct platform_trng_driver *driver,
-                            const struct device_region *device_region)
+int platform_trng_create(struct platform_trng_driver *driver, int instance)
 {
     static const struct platform_trng_iface iface =  { .poll = mock_poll };
 
-    (void)device_region;
-
+    (void)instance;
     driver->context = NULL;
     driver->iface = &iface;