i2c: nvidia-gpu: Constify the software node

Additional device properties are always just a part of a
software fwnode. If the device properties are constant, the
software node can also be constant.

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>