mtd: docg4: add missing HAS_IOMEM dependency

While building an allyesconfig for UML I received this error message(s):
 drivers/mtd/nand/docg4.c: In function 'probe_docg4':
 drivers/mtd/nand/docg4.c:1272:2: error: implicit declaration of function
 'ioremap' [-Werror=implicit-function-declaration]
 drivers/mtd/nand/docg4.c:1272:10: warning: assignment makes pointer from
 integer without a cast [enabled by default]
 drivers/mtd/nand/docg4.c:1327:2: error: implicit declaration of function
 'iounmap' [-Werror=implicit-function-declaration]

which is caused by the missing implementations on UML.

This patch adds this missing HAS_IOMEM dependency and prevents the driver from
being build on platforms with no HAS_IOMEM

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Acked-by: Mike Dunn <mikedunn@newsguy.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>