drivers: flash: stm32 page layout size differs in stm32u5 or stm32l5

Adjust the size of the stm32_flash_layout[] table depending on the
bank configuration of the stm32u5 or stm32l5 devices.
That will avoid div by zero error in flash_get_page_info()
if the layout_size is not correct.
Assign the *layout_size only once with the correct value : 3 or 1.

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