mfd: max8925: Fix mfd device register failure

we encounter rtc/power/touch driver registry failure, root cause
it is resources confilict in insert_resouce, solved by changing
mfd_add_devices 5th parameter to NULL

Signed-off-by: Qing Xu <qingx@marvell.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>