ARM: mmp: move into ARCH_MULTIPLATFORM

With all dependencies taken care of, this enables building
the Marvell mmp platform as part of ARCH_MULTIPLATFORM, along
with other ARMv5 and ARMv7 platforms.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Andrew Lunn <andrew@lunn.ch>