drivers: flash: stm32 flash base address from the DTS node

For the flash driver, the base address is the MCU internal flash
address (usually 0x8000000). This PR gets the that address
from the device tree node "st,stm32-nv-flash"
instead of relying on the CONFIG_FLASH_BASE_ADDRESS
which might differ when building for another flash memory.

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