drivers: flash: stm32 ospi flash driver waits for full chip erase

Add a function to wait for the full (bulk) Nor-octoflah erase command.
When erasing the full octo-flash, the drivers waits until
the operation is ready and the external NOR mem becomes ready itself.
The full (bulk) erase operation lasts for several seconds.

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