samples: drivers: flash_shell does not run on stm32h745/m4 boards

Do not run the samples/drivers/flash_shell application on the
stm32h745i_disco//m4 as the flash controller is not supported yet
on the M4 core (only M7). Just like the nucleo stm32h745//m4
is excluded.
Also exclude stm32h7rs disco at the moment (still TODO).

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