mfd: Fix build warning on 88pm860x

WARNING: vmlinux.o(.devinit.text+0x6c4): Section mismatch in reference
from the function device_onkey_init() to the (unknown reference)
.init.data:(unknown)
The function __devinit device_onkey_init() references a (unknown reference)
 __initdata (unknown).
If (unknown) is only used by device_onkey_init then annotate (unknown)
with a matching annotation.

It's caused by using __initdata on mfd cell resources. Replace __initdata
with __devinitdata.

Signed-off-by: Haojian Zhuang <haojian.zhuang@marvell.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>