mtd: onenand: omap2: add dependency on GPMC

OMAP2 OneNAND driver uses gpmc_omap_onenand_set_timings() provided by
OMAP_GPMC driver, so the latter cannot be module if OneNAND driver is
built-in:

  /usr/bin/arm-linux-gnueabi-ld: drivers/mtd/nand/onenand/onenand_omap2.o: in function `omap2_onenand_probe':
  onenand_omap2.c:(.text+0x520): undefined reference to `gpmc_omap_onenand_set_timings'

The OMAP_GPMC is also a runtime dependency.

Reported-by: kernel test robot <lkp@intel.com>
Fixes: 854fd920 ("memory: omap-gpmc: Allow building as a module")
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Roger Quadros <rogerq@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20221107091520.127053-1-krzysztof.kozlowski@linaro.org