mtd: gpmi: use devm_ioremap_resource

Use the devm_ioremap_resource to simplify the code.

[Note: as a side effect, this adds a missing call to request_memory().]

Signed-off-by: Huang Shijie <b32955@freescale.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>