mtd: atmel_nand: use module_platform_driver_probe()

This patch uses module_platform_driver_probe() macro which makes
the code smaller and simpler.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>