m68k: q40: Add and use "q40.h"

When building with W=1:

arch/m68k/q40/q40ints.c:84:13: warning: no previous prototype for ‘q40_init_IRQ’ [-Wmissing-prototypes]
84 | void __init q40_init_IRQ(void)
| ^~~~~~~~~~~~
arch/m68k/q40/q40ints.c:112:6: warning: no previous prototype for ‘q40_mksound’ [-Wmissing-prototypes]
112 | void q40_mksound(unsigned int hz, unsigned int ticks)
| ^~~~~~~~~~~
arch/m68k/q40/q40ints.c:152:6: warning: no previous prototype for ‘q40_sched_init’ [-Wmissing-prototypes]
152 | void q40_sched_init (void)
| ^~~~~~~~~~~~~~

Fix this by introducing a new header file "q40.h" for holding the
prototypes of functions implemented in arch/m68k/q40/.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/7f983ce079b176e1e1ac2a7d0c35b36197c5fa6b.1694613528.git.geert@linux-m68k.org