ARM: pxa: move plat-pxa to drivers/soc/

There are two drivers in arch/arm/plat-pxa: mfp and ssp. Both
of them should ideally not be needed at all, as there are
proper subsystems to replace them.

OTOH, they are self-contained and can simply be normal
SoC drivers, so move them over there to eliminate one more
of the plat-* directories.

Acked-by: Robert Jarzmik <robert.jarzmik@free.fr> (mach-pxa)
Acked-by: Lubomir Rintel <lkundrak@v3.sk> (mach-mmp)
Signed-off-by: Arnd Bergmann <arnd@arndb.de>