ARM: pick Versatile by default for !MMU

The introduction of ARCH_MULTIPLATFORM changed
the default for nommu kernels from Versatile to
Integrator, which is less common, and does not
currently build for allnoconfig because that does
not select any of the CPUs.

This also ensures that at least one of the three
board files in versatile are enabled, which lets
us successfully build an allnoconfig kernel.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Russell King <linux@arm.linux.org.uk>