staging: nvec: remove instantiating via platform device

Tegra has been converted to support device tree only. Remove support
for instantiating via platform device.

Signed-off-by: Leon Romanovsky <leon@leon.nu>
Signed-off-by: Marc Dietrich <marvin24@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>