[ARM] pxa: build arch/arm/plat-pxa/mfp.c only when PXA3xx or ARCH_MMP defined

arch/arm/plat-pxa/mfp.c is not intended to be use by PXA25x/PXA27x,
make it explicit here only for use by PXA3xx or ARCH_MMP.
Reported-by: Guennadi Liakhovestski <g.liakhovestski@gmx.de>
Reported-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Eric Miao <eric.miao@marvell.com>

arch/arm/plat-pxa/Makefile

@@ -2,6 +2,8 @@
 # Makefile for code common across different PXA processor families
 #
 
-obj-y	:= dma.o mfp.o
+obj-y	:= dma.o
 
 obj-$(CONFIG_GENERIC_GPIO)	+= gpio.o
+obj-$(CONFIG_PXA3xx)		+= mfp.o
+obj-$(CONFIG_ARCH_MMP)		+= mfp.o