dt-bindings: display: bridge: thc63lvdm83d: use standard powerdown-gpios

The name powerdown-gpios is the standard property name for the
functionality covered by the previous pwdn-gpios name. This rename
should be safe to do since the linux driver supporting the binding
(lvds-encoder.c) never implemented the property, and no dts file
names it. At least not upstream.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190111151843.11114-4-peda@axentia.se