powerpc: exclude powerbook sleep code with CONFIG_PPC64 and CONFIG_PM

We were getting powerbook sleep code included, and giving compile
errors, with CONFIG_PM=y on a 64-bit build.  This excludes that code
so the kernel will compile.  One day BenH will implement on sleep on
the G5...

Signed-off-by: Paul Mackerras <paulus@samba.org>