powerpc/amigaone: Make several functions static

These functions can all be static. Make them so. That also fixes no
previous prototype warnings:

  arch/powerpc/platforms/amigaone/setup.c:28:6: error: no previous prototype for 'amigaone_show_cpuinfo'
  arch/powerpc/platforms/amigaone/setup.c:68:13: error: no previous prototype for 'amigaone_setup_arch'
  arch/powerpc/platforms/amigaone/setup.c:86:13: error: no previous prototype for 'amigaone_init_IRQ'
  arch/powerpc/platforms/amigaone/setup.c:126:17: error: no previous prototype for 'amigaone_restart'

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240305123410.3306253-2-mpe@ellerman.id.au