drivers/sensor: iis2iclx: (FIX) enable interrupt selectively

(same considerations as commit 2f01479b)
In a multi-instance driver it may happen that on some h/w
one device should use interrupts and a second device should use
polling mode. So, CONFIG_IIS2ICLX_TRIGGER is not enough to discriminmate
if interrupt inizialization routine should be called or not; the choice
is now based whether the "irq-gpios" property is present in the DT
for that particular instance or not.

Signed-off-by: Armando Visconti <armando.visconti@st.com>