ata: pata_mpc52xx: use GFP_KERNEL

Platform_driver probe functions aren't called with locks held
and thus don't need GFP_ATOMIC. Use GFP_KERNEL instead.

Problem found with Coccinelle.

Signed-off-by: Julia Lawall <Julia.Lawall@inria.fr>
Reviewed-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>