pwm: Add ChromeOS EC PWM driver

Use the new ChromeOS EC EC_CMD_PWM_{GET,SET}_DUTY commands to control
one or more PWMs attached to the Embedded Controller. Because the EC
allows us to modify the duty cycle (as a percentage, where U16_MAX is
100%) but not the period, we assign the period a fixed value of
EC_PWM_MAX_DUTY and reject all attempts to change it.

This driver supports only device tree at the moment, because that
provides a very flexible way of describing the relationship between PWMs
and their consumer devices (e.g., backlight). On a non-DT system, we'll
probably want to use the non-GENERIC addressing (i.e., we'll need to
make special device instances that will use EC_PWM_TYPE_KB_LIGHT or
EC_PWM_TYPE_DISPLAY_LIGHT), as well as the relatively inflexible
pwm_lookup infrastructure for matching devices. Defer that work for now.

Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

drivers/pwm/Kconfig

@@ -147,6 +147,13 @@ config PWM_CRC
 	  Generic PWM framework driver for Crystalcove (CRC) PMIC based PWM
 	  control.
 
+config PWM_CROS_EC
+	tristate "ChromeOS EC PWM driver"
+	depends on MFD_CROS_EC
+	help
+	  PWM driver for exposing a PWM attached to the ChromeOS Embedded
+	  Controller.
+
 config PWM_EP93XX
 	tristate "Cirrus Logic EP93xx PWM support"
 	depends on ARCH_EP93XX