drivers: flash: stm32h7 with RDP protection

Implement the readout protection for the STM32H7 series.
Define the specific functions used by the flash_stm32h7.
Move the stm32h7 flash register manipulation in a write_optb()
to be called by the flash_stm32_set_rdp_level.

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