powerpc: Add the PC speaker only when requested

This eliminates this minor boot-time debugging error message:

[    1.316451] calling  add_pcspkr+0x0/0x84
[    1.316478] initcall add_pcspkr+0x0/0x84 returned -19 after 0 msecs

Signed-off-by: Emil Medve <Emilian.Medve@Freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>