mtd: mxc_nand: use __iowrite32_copy for 32 bit copy

The driver was also using own method to do 32bit copy, turns out
we have a kernel API so use that instead

Signed-off-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>