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

When building with W=1:

    arch/m68k/mac/iop.c:235:13: warning: no previous prototype for ‘iop_init’ [-Wmissing-prototypes]
      235 | void __init iop_init(void)
	  |             ^~~~~~~~
    arch/m68k/mac/via.c:112:13: warning: no previous prototype for ‘via_init’ [-Wmissing-prototypes]
      111 | void __init via_init(void)
	  |             ^~~~~~~~
    arch/m68k/mac/via.c:623:13: warning: no previous prototype for ‘via_init_clock’ [-Wmissing-prototypes]
      593 | void __init via_init_clock(void)
	  |             ^~~~~~~~~~~~~~
    arch/m68k/mac/oss.c:37:13: warning: no previous prototype for ‘oss_init’ [-Wmissing-prototypes]
       37 | void __init oss_init(void)
	  |             ^~~~~~~~
    arch/m68k/mac/psc.c:76:13: warning: no previous prototype for ‘psc_init’ [-Wmissing-prototypes]
       76 | void __init psc_init(void)
	  |             ^~~~~~~~
    arch/m68k/mac/baboon.c:25:13: warning: no previous prototype for ‘baboon_init’ [-Wmissing-prototypes]
       25 | void __init baboon_init(void)
	  |             ^~~~~~~~~~~
    arch/m68k/mac/macboing.c:155:6: warning: no previous prototype for ‘mac_mksound’ [-Wmissing-prototypes]
      155 | void mac_mksound( unsigned int freq, unsigned int length )
	  |      ^~~~~~~~~~~
    arch/m68k/mac/misc.c:608:5: warning: no previous prototype for ‘mac_hwclk’ [-Wmissing-prototypes]
      608 | int mac_hwclk(int op, struct rtc_time *t)
	  |     ^~~~~~~~~

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

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/d1fe0014a9e472a305333de4fa17f335c93d73af.1694613528.git.geert@linux-m68k.org