dt-bindings: mfd: LMU: Add the ramp up/down property

Document the ramp-up and ramp-down property in the binding.
Removing the "sec" from the property definition as seconds is
implied.

Signed-off-by: Dan Murphy <dmurphy@ti.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>