stm32_iwdg: rework early interrupt to dump core registers

Rework the early interrupt to dump core registers in debug mode.
In release mode, it will clear the interrupt and wait watchdog
to expire.

Signed-off-by: Lionel Debieve <lionel.debieve@st.com>
Change-Id: I0b690d30f5b52b6fc708fe440b1c15bd3b3f341d
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/186541


Reviewed-by: CITOOLS <smet-aci-reviews@lists.codex.cro.st.com>
Reviewed-by: Yann GAUTIER <yann.gautier@st.com>