[POWERPC] Don't allow PMAC_APM_EMU for 64-bit

In b3028878

 I switched the apm emulation
code to use the generic code but accidentally dropped the PPC32
dependency from the configuration symbol. This adds it back.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Paul Mackerras <paulus@samba.org>