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 <jason@lakedaemon.net>
Cc: Andrew Lunn <andrew@lunn.ch>
Cc: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Cc: Gregory Clement <gregory.clement@bootlin.com>
Cc: Russell King <linux@armlinux.org.uk>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>