mtd: gpmi: Use devm_kzalloc()

Using devm_kzalloc() can make the code simpler.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Huang Shijie <b32955@freescale.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>