UPSTREAM: dt-bindings: pwm: Add binding for ChromeOS EC PWM

UPSTREAM: dt-bindings: pwm: Add binding for ChromeOS EC PWM

The ChromeOS Embedded Controller can support controlling its attached
PWMs via its host-command interface. The number of supported PWMs varies
on a per-board basis, but we can autodetect this by checking the error
codes, so we don't need an extra property for this. And because the EC
only allows specifying the duty cycle and not the period, we don't
specify the period via pwm-cells, and instead have only support for one
cell -- to specify the index.

Signed-off-by: Brian Norris <briannorris@chromium.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>
(cherry picked from commit 9e60f50b4a79ae2df791d89d08cf2b78ad7629bd)

Change-Id: Ibb2ac5cff1e8cc2ab43c9f1f89e68e48da23d897
Signed-off-by: David Wu <david.wu@rock-chips.com>