drm/bridge: ti-sn65dsi86: Implement the pwm_chip

The SN65DSI86 provides the ability to supply a PWM signal on GPIO 4,
with the primary purpose of controlling the backlight of the attached
panel. Add an implementation that exposes this using the standard PWM
framework, to allow e.g. pwm-backlight to expose this to the user.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20211025170925.3096444-3-bjorn.andersson@linaro.org