staging: nvec: move device tree parsing to its own function

This moves device tree parsing to its own function so more stuff can
be cleanly added in the future.

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>