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>

arch/arm/mach-at91/setup.c

@@ -87,7 +87,7 @@ void __init at91_init_sram(int bank, unsigned long base, unsigned int length)
 	iotable_init(desc, 1);
 }
 
-static struct map_desc at91_io_desc __initdata = {
+static struct map_desc at91_io_desc __initdata __maybe_unused = {
 	.virtual	= AT91_VA_BASE_SYS,
 	.pfn		= __phys_to_pfn(AT91_BASE_SYS),
 	.length		= SZ_16K,