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

When building with W=1:

arch/m68k/amiga/amisound.c:48:13: warning: no previous prototype for ‘amiga_init_sound’ [-Wmissing-prototypes]
48 | void __init amiga_init_sound(void)
| ^~~~~~~~~~~~~~~~
arch/m68k/amiga/amisound.c:71:6: warning: no previous prototype for ‘amiga_mksound’ [-Wmissing-prototypes]
71 | void amiga_mksound( unsigned int hz, unsigned int ticks )
| ^~~~~~~~~~~~~

Fix this by introducing a new header file "amiga.h" for holding the
prototypes of functions implemented in arch/m68k/amiga/.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/7e9395e1de1c9a34d16ea40a522057f05bbee400.1694613528.git.geert@linux-m68k.org