[POWERPC] Fix build breakage with CONFIG_PPC32

low_cpu_die is called from the CPU hotplug code on 32-bit powermacs,
but it is only defined if CONFIG_PM || CONFIG_CPU_FREQ_PMAC.  This
changes the ifdef so it is defined for CONFIG_HOTPLUG_CPU on 32-bit
machines.

Signed-off-by: Srinivasa DS <srinivasa@in.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>