mtd: orion_nand.c: add error handling and use resource_size()

Use platform_get_resource() to fetch the memory resource and
add error handling for when it is missing. Use resource_size()
for the ioremap().

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