ARM: at91: skip at91_io_desc definition for NOMMU

On NOMMU systems, we do cannot remap the MMIO space, so the
definition of at91_io_desc is unused.

Without this patch, building at91x40_defconfig results in:

arch/arm/mach-at91/setup.c:90:24: warning: 'at91_io_desc' defined but not used [-Wunused-variable]

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: Jean-Christophe Plagniol-Villard <plagnioj@jcrosoft.com>