mtd: denali_dt: Use devm_ioremap_resource()

Use devm_ioremap_resource() in order to make the code
simpler, and remove redundant return value check of
platform_get_resource_byname() because the value is
checked by devm_ioremap_resource().

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Tested-by: Dinh Nguyen <dinguyen@altera.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>