staging: nvec: convert to use platform register and mfdcells

This patch converts the nvec to use mfd cells and improves the
registration of the platform driver. The child drivers are also
converted to use mfd cells and platform registration.

Signed-off-by: Marc Dietrich <marvin24@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>