dt-bindings: display: panel: Make reset-gpio as optional for Raydium RM67200

Although the datasheet of the panel module describes that it has a
reset pin, in the actual hardware design, we often use an RC circuit
to control the reset, and rarely use GPIO to control the reset. This
is the way it is done on our numerous development boards (such as RK3568,
RK3576 EVB).
So make the reset-gpio optional.

Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20250616070536.670519-1-andyshrk@163.com