gpio/tegra: dt: add binding for gpio polarity

Allocate one bit in the available extra cell to indicate if the gpio
should be considered logically inverted.

Signed-off-by: Olof Johansson <olof@lixom.net>
Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>