[ARM] mmp: fix build failure due to IRQ_PMU depends on ARCH_PXA

PMU is not tested and enabled on MMP architecture at this moment,
the device IRQ number, IRQ_PMU depends on ARCH_PXA. Build PMU only
for ARCH_PXA.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>