ARM: XEN depends on having a MMU

arch/arm/xen/enlighten.c (and maybe others) use MMU-specific functions
like pte_mkspecial which are only available on MMU builds. So let XEN
depend on MMU.

Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>