spi: mediatek: single device does not require cs_gpios

When only one device is present, it is not necessary to specify
cs_gpios, as the CS line can be controlled by the hardware
module.

Without this patch, older device tree bindings used before
37457607 "spi: mediatek: mt8173 spi multiple devices support"
would cause a panic on boot. This fixes the crash, and
re-introduces backward compatibility.

Signed-off-by: Nicolas Boichat <drinkcat@chromium.org>
Acked-by: Leilk Liu <leilk.liu@mediatek.com>
Signed-off-by: Mark Brown <broonie@kernel.org>