rpi3: Move rng driver to drivers

To allow sharing the driver between the RPi3 and RPi4, move the random
number generator driver into the generic driver directory.

Change-Id: Iae94d7cb22c6bce3af9bff709d76d4caf87b14d1
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
diff --git a/plat/rpi/rpi3/rpi3_private.h b/plat/rpi/rpi3/rpi3_private.h
index 53078f8..b01c40c 100644
--- a/plat/rpi/rpi3/rpi3_private.h
+++ b/plat/rpi/rpi3/rpi3_private.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2015-2018, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2015-2019, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -33,9 +33,6 @@
 /* IO storage utility functions */
 void plat_rpi3_io_setup(void);
 
-/* Hardware RNG functions */
-void rpi3_rng_read(void *buf, size_t len);
-
 /* VideoCore firmware commands */
 int rpi3_vc_hardware_get_board_revision(uint32_t *revision);