m68k: mvme16x: Add and use "mvme16x.h"

When building with W=1:

    arch/m68k/mvme16x/config.c:208:6: warning: no previous prototype for ‘mvme16x_cons_write’ [-Wmissing-prototypes]
      208 | void mvme16x_cons_write(struct console *co, const char *str, unsigned count)
	  |      ^~~~~~~~~~~~~~~~~~

Fix this by introducing a new header file "mvme16x.h" for holding the
prototypes of functions implemented in arch/m68k/mvme16x/.
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/6200cc3b26fad215c4524748af04692e38c5ecd2.1694613528.git.geert@linux-m68k.org

arch/m68k/kernel/early_printk.c

@@ -12,8 +12,8 @@
 #include <linux/string.h>
 #include <asm/setup.h>
 
-extern void mvme16x_cons_write(struct console *co,
-			       const char *str, unsigned count);
+
+#include "../mvme16x/mvme16x.h"
 
 asmlinkage void __init debug_cons_nputs(const char *s, unsigned n);
 

arch/m68k/mvme16x/config.c

@@ -38,6 +38,8 @@
 #include <asm/mvme16xhw.h>
 #include <asm/config.h>
 
+#include "mvme16x.h"
+
 extern t_bdid mvme_bdid;
 
 static MK48T08ptr_t volatile rtc = (MK48T08ptr_t)MVME_RTC_BASE;

arch/m68k/mvme16x/mvme16x.h

+/* SPDX-License-Identifier: GPL-2.0-only */
+
+struct console;
+
+/* config.c */
+void mvme16x_cons_write(struct console *co, const char *str, unsigned count);