staging/nvec: Do not pass resource to mfd_add_devices()

The mfd_add_devices() function takes a struct resource * as fifth
argument, but the nvec driver passes in a void __iomem *. The driver
gets away with it because none of the subdevices ever directly access
the registers.

Since subdevices never need to access the registers we can simply pass
NULL instead.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Acked-by: Marc Dietrich <marvin24@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>