boards: st: nucleo_l552ze_q: Populate ns flash partitions

Rather than providing flash partitions in samples, provide default
partition (no tests) as part of the board ns variant default configuration.

Update or remove related overlays.

Update ns variant yaml file to specify actual flash available for the
ns application (36K).

Additionaly, fix comment in b_u585i_iot02a_ns.

Signed-off-by: Erwan Gouriou <erwan.gouriou@st.com>