rpi3: Use new console APIs

Switch to the new console APIs enabled by setting MULTI_CONSOLE_API=1.

The crash console doesn't use this API, it uses internally the core
functions of the 16550 console.

`bl31_plat_runtime_setup` is no longer needed. When this platform port
was introduced, that function used to disable the console. It was needed
to override that behaviour. The new behaviour is to switch to the
runtime console. The console is registered for all scopes (boot, crash
and runtime) in `rpi3_console_init` so it is not needed to override the
default behaviour anymore.

Update documentation.

Change-Id: If2ee8f91044216183b7ef142e5c05ad6220ae92f
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>
diff --git a/plat/rpi3/rpi3_private.h b/plat/rpi3/rpi3_private.h
index 01c4055..a9fbfe4 100644
--- a/plat/rpi3/rpi3_private.h
+++ b/plat/rpi3/rpi3_private.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2015-2017, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2015-2018, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -14,6 +14,7 @@
  ******************************************************************************/
 
 /* Utility functions */
+void rpi3_console_init(void);
 void rpi3_setup_page_tables(uintptr_t total_base, size_t total_size,
 			    uintptr_t code_start, uintptr_t code_limit,
 			    uintptr_t rodata_start, uintptr_t rodata_limit