kernel: hooks: make definitions appear in Doxygen

The hooks' signature was not visible to Doxygen due to preprocessor
conditionals - instead, the dummy no-op implementation was seen instead.

Update #ifdef guard to make the hooks' signature and the associated
documentation visible to Doxygen such that it appears in docs.

Also fix a tiny typo on soc_reset_hook()'s documentation comment.

Signed-off-by: Mathieu Choplain <mathieu.choplain-ext@st.com>