drivers: flash for the stm32l5x or stm32u5 serie with max 2MB

Adds the stm32u5 flash controller driver for this serie
to the existing stm32l5 flash driver part
Only 1 or 2 MB devices exist today (4MB is possible in the future).
This flash controller driver is adapted from the flash_stm32l5.c

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