mtd: nand: oxnas_nand: fix build errors on arch/um, require HAS_IOMEM

Fix build errors on arch/um, which does not support HAS_IOMEM,
while the oxnas_nand.c driver uses interfaces that are
supplied by HAS_IOMEM.

(loadable module build:)
ERROR: "devm_ioremap_resource" [drivers/mtd/nand/oxnas_nand.ko] undefined!
or (built-in build:)
drivers/built-in.o: In function `oxnas_nand_probe':
drivers/mtd/nand/oxnas_nand.c:102: undefined reference to `devm_ioremap_resource'

Fixes: 66859249 ("mtd: nand: Add OX820 NAND Support")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>