mtd: fsl_ifc_nand: use more portable i/o accessors

in/out_be32 accessors are Power arch centric whereas
ioread/writebe32 are available in other arches.

Since the IFC device registers are annotated big endian in
fsl_ifc.h, the accessor annotations now match, resulting in the
pleasant side-effect of this patch silencing sparse endian
warnings such as the following:

drivers/mtd/nand/fsl_ifc_nand.c:179:19: warning: incorrect type in argument 1 (different base types)
drivers/mtd/nand/fsl_ifc_nand.c:179:19:    expected unsigned int volatile [noderef] [usertype] <asn:2>*addr
drivers/mtd/nand/fsl_ifc_nand.c:179:19:    got restricted __be32 [noderef] <asn:2>*<noident>

Signed-off-by: Kim Phillips <[email protected]>
Signed-off-by: Artem Bityutskiy <[email protected]>