ARM: OMAP2: Fix section warning for n8x0 when CONFIG_MMC_OMAP is not set

Otherwise we get:

arch/arm/mach-omap2/board-n8x0.c:39:12: warning:
'slot1_cover_open' defined but not used [-Wunused-variable]
arch/arm/mach-omap2/board-n8x0.c:40:12: warning:
'slot2_cover_open' defined but not used [-Wunused-variable]
arch/arm/mach-omap2/board-n8x0.c:41:23: warning:
'mmc_device' defined but not used [-Wunused-variable]

Reported-by: Russell King <linux@arm.linux.org.uk>
Signed-off-by: Tony Lindgren <tony@atomide.com>