samples: drivers: stm32h7 octospi enables DMA

Adds the dma transfer for the octoSPI on NOR octo Flash
of the stm32h7b3i and stm32h735g disco kits.
The channel for the MDMA is 0-15.
The MDMA request is 0x16 for the OCTOSPI1 fifo threshold

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