[ARM] pxa: move pmu device back into mach-pxa/

Not tested and enabled on MMP at this moment, and since the IRQ
is different from mach-pxa, I'd prefer to move the PMU device
back into mach-pxa/. Will introduce the PMU device to MMP once
it's enabled and tested.

Signed-off-by: Eric Miao <eric.y.miao@gmail.com>

arch/arm/mach-pxa/devices.h

@@ -1,3 +1,4 @@
+extern struct platform_device pxa_device_pmu;
 extern struct platform_device pxa_device_mci;
 extern struct platform_device pxa3xx_device_mci2;
 extern struct platform_device pxa3xx_device_mci3;