The EC_CMD_PWM_{GET,SET}_DUTY commands allow us to control a PWM that is
attached to the EC, rather than the main host SoC. The API provides
functionality-based (e.g., keyboard light, backlight) or index-based
addressing of the PWM(s). Duty cycles are represented by a 16-bit value,
where 0 maps to 0% duty cycle and U16_MAX maps to 100%. The period
cannot be controlled.

This command set is more generic than, e.g.,
EC_CMD_PWM_{GET,SET}_KEYBOARD_BACKLIGHT and could possibly used to
replace it on future products.

Let's update the command header to include the definitions.
Signed-off-by: NBrian Norris <briannorris@chromium.org>
Acked-by: NLee Jones <lee.jones@linaro.org>
Signed-off-by: NThierry Reding <thierry.reding@gmail.com>