m68k/amiga/: possible cleanups

This patch contains the following possible cleanups:
- amiints.c: add a proper prototype for amiga_init_IRQ() in
             include/asm-m68k/amigaints.h
- make the following needlessly global code static:
  - config.c: amiga_model
  - config.c: amiga_psfreq
  - config.c: amiga_serial_console_write()
- #if 0 the following unused functions:
  - config.c: amiga_serial_puts()
  - config.c: amiga_serial_console_wait_key()
  - config.c: amiga_serial_gets()
- remove the following unused variable:
  - config.c: amiga_masterclock

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>