boards: shields: x_nucleo_iks01a1: '-' is not allowed in nodelabels

When moving nodelabels to <device_name>_<shields_name>, '-' included
in <device_name> was then introduced in the  device nodelabel.
But '-' isn't an allowed nodelabel character, hence this is breaking
compilation with those shields.
Convert '-' to '_'.

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