boards: arm: b_l4s5i_iot01a overlapping flash partitions

In this board DT the "storage" and "image-scratch" are declared
with overlapping address spaces.
As a result, when one of these partitions is used it can corrupt the
contents of the other one. This is the case when mcuboot performs an
image swap using the scratch partition.

To fix the bug, the size of the scratch partition is reduced.


Signed-off-by: Francois Ramu <francois.ramu@st.com>