mtd: fsl_ifc_nand: Use void type for IFC buffer

The IFC buffer is accessed via 8-bit and 16-bit accessors. Changing
the 'addr' member of 'struct fsl_ifc_nand_ctrl' from 'u8 __iomem *' to
'void __iomem *' eliminates the need for explicit casts when the
16-bit accessors are used.

Signed-off-by: Aaron Sierra <asierra@xes-inc.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>