[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>