ARM: OMAP2+: gpmc: annotate exit sections properly

compiler complained,
`gpmc_remove' referenced in section `.data' of arch/arm/mach-omap2/built-in.o: defined in discarded section `.exit.text' of arch/arm/mach-omap2/built-in.o

Annotate gpmc_remove function and dependents with __devexit.

Reported-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Afzal Mohammed <afzal@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>