mtd: onenand: samsung: Fix iomem access with regular memcpy

The __iomem memory should be copied with memcpy_fromio.  This fixes
Sparse warnings like:

    drivers/mtd/nand/onenand/samsung_mtd.c:678:40: warning: incorrect type in argument 2 (different address spaces)
    drivers/mtd/nand/onenand/samsung_mtd.c:678:40:    expected void const *from
    drivers/mtd/nand/onenand/samsung_mtd.c:678:40:    got void [noderef] <asn:2> *[assigned] p
    drivers/mtd/nand/onenand/samsung_mtd.c:679:19: warning: incorrect type in assignment (different address spaces)
    drivers/mtd/nand/onenand/samsung_mtd.c:679:19:    expected void [noderef] <asn:2> *[assigned] p
    drivers/mtd/nand/onenand/samsung_mtd.c:679:19:    got unsigned char *

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>