[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>