mtd: omap2: 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()
because the value is checked by devm_ioremap_resource(). Also,
'unsigned long mem_size' is removed from 'struct omap_nand_info',
because the 'mem_size' variable is not necessary anymore.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>