mtd: nand/gpio: use io{read,write}*_rep accessors

The {read,write}s{b,w,l} operations are not defined by all
architectures and are being removed from the asm-generic/io.h
interface.

This patch replaces the usage of these string functions in the mtd
gpio accessors with io{read,write}{8,16,32}_rep calls instead.

Signed-off-by: Matthew Leach <matthew@mattleach.net>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>