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

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