dts/binding/sensor: Fix ST sensors irq-gpios property being required

Change binding for ST sensors property 'irq-gpios' to optional for the
cases of in which its obvious from the driver that the property is
optional (there's an ifdef based on the #define
DT_INST_0_ST_LIS2DH_IRQ_GPIOS_*).

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