ARM: ux500: U8500 depends on MMU

There is no way to build  U8500 kernels without an MMU
at this point because of dependencies on MMU-only functions.

As long as nobody is interested in fixing this, let's just disable
the platforms for nommu kernels.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>