[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>

arch/arm/plat-pxa/Makefile

@@ -2,8 +2,9 @@
 # Makefile for code common across different PXA processor families
 #
 
-obj-y	:= dma.o pmu.o
+obj-y	:= dma.o
 
+obj-$(CONFIG_ARCH_PXA)		+= pmu.o
 obj-$(CONFIG_GENERIC_GPIO)	+= gpio.o
 obj-$(CONFIG_PXA3xx)		+= mfp.o
 obj-$(CONFIG_ARCH_MMP)		+= mfp.o