feat(dlog): adopt FF-A in `stdout_putchar`

Use the extended FFA_CONSOLE_LOG64 ABI in `stdout_putchar` when it is
available.

Change-Id: I66405a7082b8af484d6829c9f6cea9209e3b66ae
Signed-off-by: Karl Meakin <karl.meakin@arm.com>
diff --git a/inc/hf/stdout.h b/inc/hf/stdout.h
index 73b667d..2405057 100644
--- a/inc/hf/stdout.h
+++ b/inc/hf/stdout.h
@@ -8,10 +8,13 @@
 
 #pragma once
 
+#include <stdint.h>
+
 /**
  * Print one character to standard output.
  * This is intentionally called differently from functions in <stdio.h> so as to
  * avoid clashes when linking against libc.
  */
+void stdout_init(uint32_t ffa_version);
 void stdout_putchar(char c);
 void stdout_flush(void);