modules: hal_nordic: nrfx_glue: Fix unused argument

NRFX_IRQ_PRIORITY_SET is defined as empty. This causes an unused
argument warning in many callers (e.g. nrfy_gpiote_int_init). Fix it by
using ARG_UNUSED for the priority argument.

Signed-off-by: Marco Widmer <marco.widmer@bytesatwork.ch>