dt: Document: correct tegra20/30 pinctrl slew-rate name

change nvidia,slew_rate* to nvidia,slew-rate*

Cc: stable@kernel.org
Signed-off-by: Pritesh Raithatha <praithatha@nvidia.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Tested-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>