drm/bridge: ti-sn65dsi86: Associate PWM device to auxiliary device

It's the ti_sn65dsi86.pwm auxiliary driver that creates the pwmchip, so
let the auxiliary device be the parent of the pwm device.

Note that getting a reference to the ti-sn65dsi86's pwm using pwm_get()
isn't affected by this change as ti_sn65dsi86_add_aux_device() sets the
auxiliary device's of_node to that of the main device.

Also change PM runtime tracking and diagnostic messages to use that one.
After enabling runtime PM operation for the auxiliary device, all works
as expected as parent devices are handled just fine.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Tested-by: Nikita Travkin <nikita@trvn.ru> # Acer Aspire 1
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20231209153108.1988551-2-u.kleine-koenig@pengutronix.de