samples/shields: x_nucleo_iks01a3: Add config for stm32mp157c_dk2

Shield x_nucleo_iks01a3 could be used with board stm32mp157c_dk2,
but it requires not using the irq-gpio pin.
Provide a specific configuration for this board that does not make
use of this pin, sot set all sensors' triggers mode to NONE.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>