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>