mtd: plat_nand: request memory resource before doing ioremap

Add a request_mem_region() before doing the ioremap().  Also, use the
resource_size macro instead of doing the end - start + 1 calc by hand.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>