Don't compile the PMU power driver on 64-bit PowerPC

As reported by Stephen Rothwell, an allmodconfig build on 64-bit
PowerPC reports these errors:

ERROR: "pmu_batteries" [drivers/power/pmu_battery.ko] undefined!
ERROR: "pmu_battery_count" [drivers/power/pmu_battery.ko] undefined!
ERROR: "pmu_power_flags" [drivers/power/pmu_battery.ko] undefined!

This fixes the problem by not building pmu_battery.ko on ppc64.  There
are no battery-powered ppc64 machines with an Apple PMU, and we can be
reasonably confident there never will be.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>