drivers: flash: stm32 flash driver has a Kconfig STM32_MEMMAP

This CONFIG_STM32_MEMMAP is for enabling the MemoryMapped mode
on external octo or quad spi memory.
In this case, the flash_stm32_read is done in mem map mode
the flash_stm32_erase is not available.

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