mtd: spear_smi: add missing __iomem annotation

Added missing __iomem annotation in order to fix the following
sparse warnings:

drivers/mtd/devices/spear_smi.c:566:13: warning: incorrect type in assignment (different address spaces)
drivers/mtd/devices/spear_smi.c:566:13:    expected void *src
drivers/mtd/devices/spear_smi.c:566:13:    got void [noderef] <asn:2>*
drivers/mtd/devices/spear_smi.c:586:9: warning: incorrect type in argument 2 (different address spaces)
drivers/mtd/devices/spear_smi.c:586:9:    expected void const volatile [noderef] <asn:2>*<noident>
drivers/mtd/devices/spear_smi.c:586:9:    got unsigned char [usertype] *<noident>
drivers/mtd/devices/spear_smi.c:659:14: warning: incorrect type in assignment (different address spaces)
drivers/mtd/devices/spear_smi.c:659:14:    expected void *dest
drivers/mtd/devices/spear_smi.c:659:14:    got void [noderef] <asn:2>*
drivers/mtd/devices/spear_smi.c:620:9: warning: incorrect type in argument 1 (different address spaces)
drivers/mtd/devices/spear_smi.c:620:9:    expected void volatile [noderef] <asn:2>*<noident>
drivers/mtd/devices/spear_smi.c:620:9:    got void *dest
drivers/mtd/devices/spear_smi.c:620:9: warning: incorrect type in argument 1 (different address spaces)
drivers/mtd/devices/spear_smi.c:620:9:    expected void volatile [noderef] <asn:2>*<noident>
drivers/mtd/devices/spear_smi.c:620:9:    got void *dest
drivers/mtd/devices/spear_smi.c:620:9: warning: incorrect type in argument 1 (different address spaces)
drivers/mtd/devices/spear_smi.c:620:9:    expected void volatile [noderef] <asn:2>*<noident>
drivers/mtd/devices/spear_smi.c:620:9:    got void *dest

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>