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

When building with W=1:

    arch/m68k/apollo/dn_ints.c:43:13: warning: no previous prototype for ‘dn_init_IRQ’ [-Wmissing-prototypes]
       43 | void __init dn_init_IRQ(void)
	  |             ^~~~~~~~~~~

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

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/d78eceb83b8dd1931be1789204898060664e23f6.1694613528.git.geert@linux-m68k.org