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

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

When building with W=1:

    arch/m68k/atari/time.c:59:1: warning: no previous prototype for ‘atari_sched_init’ [-Wmissing-prototypes]
       59 | atari_sched_init(void)
	  | ^~~~~~~~~~~~~~~~
    arch/m68k/atari/time.c:140:5: warning: no previous prototype for ‘atari_mste_hwclk’ [-Wmissing-prototypes]
      140 | int atari_mste_hwclk( int op, struct rtc_time *t )
	  |     ^~~~~~~~~~~~~~~~
    arch/m68k/atari/time.c:199:5: warning: no previous prototype for ‘atari_tt_hwclk’ [-Wmissing-prototypes]
      199 | int atari_tt_hwclk( int op, struct rtc_time *t )
	  |     ^~~~~~~~~~~~~~
    arch/m68k/atari/ataints.c:267:13: warning: no previous prototype for ‘atari_init_IRQ’ [-Wmissing-prototypes]
      267 | void __init atari_init_IRQ(void)
	  |             ^~~~~~~~~~~~~~
    arch/m68k/atari/atasound.c:36:6: warning: no previous prototype for ‘atari_microwire_cmd’ [-Wmissing-prototypes]
       36 | void atari_microwire_cmd (int cmd)
	  |      ^~~~~~~~~~~~~~~~~~~
    arch/m68k/atari/atasound.c:53:6: warning: no previous prototype for ‘atari_mksound’ [-Wmissing-prototypes]
       53 | void atari_mksound (unsigned int hz, unsigned int ticks)
	  |      ^~~~~~~~~~~~~

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

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/336e4a478ccbfd7e3e91cdbd27636947587a23a6.1694613528.git.geert@linux-m68k.org