tests: drivers: flash driver for the non-secure stm32u5 board

this commit adds an overlay to run the tests/drivers/flash
on the b_u585i_iot02a_ns target platform.
It renames the non-secure partition to slot1_ns_partition
to match the testcases.

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