drivers/panel: raydium-rm67200: Make reset-gpio optional

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>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20250616070536.670519-2-andyshrk@163.com