mtd: impa7: add missing __iomem annotation

Added missing __iomem annotation and used NULL instead of 0
in order to fix the following sparse warnings:

drivers/mtd/maps/impa7.c:82:32: warning: Using plain integer as NULL pointer
drivers/mtd/maps/impa7.c:96:34: warning: cast removes address space of expression
drivers/mtd/maps/impa7.c:96:34: warning: incorrect type in argument 1 (different address spaces)
drivers/mtd/maps/impa7.c:96:34:    expected void volatile [noderef] <asn:2>*addr
drivers/mtd/maps/impa7.c:96:34:    got void *<noident>
drivers/mtd/maps/impa7.c:108:34: warning: cast removes address space of expression
drivers/mtd/maps/impa7.c:108:34: warning: incorrect type in argument 1 (different address spaces)
drivers/mtd/maps/impa7.c:108:34:    expected void volatile [noderef] <asn:2>*addr
drivers/mtd/maps/impa7.c:108:34:    got void *<noident>
drivers/mtd/maps/impa7.c:109:45: warning: Using plain integer as NULL pointer

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>