board: stm32mp1: Keep error led ON in case of low power detection

Since commit commit dd281085 ("stm32mp1: board: support of
error led on ed1/ev1 board") the attended behavior was no more
respected in case of low power source detection on DK2.

The expected behavior is either the error LED keeps blinking for ever,
or blinks 2 or 3 times and must stay ON.

Reviewed-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>