ARM: OMAP2+: GPMC: Export gpmc_enable_hwecc and gpmc_calculate_ecc

To be able to compile kernel/drivers/mtd/nand/omap2.ko as module, that
two symbols need to be exported. Otherwise, I get following error
message

    ERROR: "gpmc_calculate_ecc" [drivers/mtd/nand/omap2.ko] undefined!
    ERROR: "gpmc_enable_hwecc" [drivers/mtd/nand/omap2.ko] undefined!

Signed-off-by: Bernhard Walle <walle@corscience.de>
Signed-off-by: Tony Lindgren <tony@atomide.com>