arm: mach-dove: Mark dove_io_desc as __maybe_unused

Without this, we get the warnings below when CONFIG_MMU is disabled:

linux/arch/arm/mach-dove/common.c:51:24: warning: ‘dove_io_desc’ defined
but not used [-Wunused-variable]
static struct map_desc dove_io_desc[] __initdata = {
^~~~~~~~~~~~

Cc: Jason Cooper <[e-mail]>
Cc: Andrew Lunn <[e-mail]>
Cc: Sebastian Hesselbarth <[e-mail]>
Cc: Gregory Clement <[e-mail]>
Cc: Russell King <[e-mail]>
Signed-off-by: Vincenzo Frascino <[e-mail]>
Signed-off-by: Gregory CLEMENT <[e-mail]>

arch/arm/mach-dove/common.c

···
 /*****************************************************************************
  * I/O Address Mapping
  ****************************************************************************/
-static struct map_desc dove_io_desc[] __initdata = {
+static struct map_desc __maybe_unused dove_io_desc[] __initdata = {
 	{
 		.virtual	= (unsigned long) DOVE_SB_REGS_VIRT_BASE,
 		.pfn		= __phys_to_pfn(DOVE_SB_REGS_PHYS_BASE),