ARM: shmobile: mark mackerel sh_mmcif_device __maybe_unused

Patch eac036ef9e "ARM: shmobile: streamline mackerel SD and MMC devices"
made the use of the sh_mmcif_device variable for mackarel optional,
but the definition is always provided, causing a build warning.

arch/arm/mach-shmobile/board-mackerel.c:1120:31: warning: 'sh_mmcif_device'
	defined but not used [-Wunused-variable]

Marking the variable as __maybe_unused will do the right thing here.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>

arch/arm/mach-shmobile/board-mackerel.c

@@ -1118,7 +1118,7 @@ static struct sh_mmcif_plat_data sh_mmcif_plat = {
 	.slave_id_rx	= SHDMA_SLAVE_MMCIF_RX,
 };
 
-static struct platform_device sh_mmcif_device = {
+static struct platform_device sh_mmcif_device __maybe_unused = {
 	.name		= "sh_mmcif",
 	.id		= 0,
 	.dev		= {