dts/bindings: st: sensors: Mark 'irq-gpios' as optional

The 'irq-gpios' property is optional as the drivers work fine if this
property isn't set.  The property is only required if "TRIGGER" mode is
enabled in the drivers.

As such mark 'irq-gpios' as 'required:false`.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>