drm/panel: sitronix-st7789v: make reset GPIO optional

The reset pin might not be software controllable from the SoC,
so make it optional.

Reviewed-by: Michael Riesch <michael.riesch@wolfvision.net>
Signed-off-by: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230714013756.1546769-6-sre@kernel.org